bpo-41710: Add pytime_add() and pytime_mul()

[vstinner]

Add pytime_add() and pytime_mul() functions to pytime.c to compute
t+t2 and t*k with clamping to [_PyTime_MIN; _PyTime_MAX].

https://bugs.python.org/issue41710

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot AMD64 Windows10 Pro 3.x has failed when building commit d62d925.

What do you need to do:

1. Don't panic.
2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
3. Go to the page of the buildbot that failed (https://buildbot.python.org/all/#builders/596/builds/1078) and take a look at the build logs.
4. Check if the failure is related to this commit (d62d925) or if it is a false positive.
5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/all/#builders/596/builds/1078

Summary of the results of the build (if available):

Click to see traceback logs

remote: Enumerating objects: 13, done.        
remote: Counting objects:   7% (1/13)        
remote: Counting objects:  15% (2/13)        
remote: Counting objects:  23% (3/13)        
remote: Counting objects:  30% (4/13)        
remote: Counting objects:  38% (5/13)        
remote: Counting objects:  46% (6/13)        
remote: Counting objects:  53% (7/13)        
remote: Counting objects:  61% (8/13)        
remote: Counting objects:  69% (9/13)        
remote: Counting objects:  76% (10/13)        
remote: Counting objects:  84% (11/13)        
remote: Counting objects:  92% (12/13)        
remote: Counting objects: 100% (13/13)        
remote: Counting objects: 100% (13/13), done.        
remote: Compressing objects: 100% (1/1)        
remote: Compressing objects: 100% (1/1), done.        
remote: Total 7 (delta 6), reused 6 (delta 6), pack-reused 0        
From https://github.com/python/cpython
 * branch                  main       -> FETCH_HEAD
Note: switching to 'd62d925823b005c33b432e527562b573a3a89635'.

You are in 'detached HEAD' state. You can look around, make experimental
changes and commit them, and you can discard any commits you make in this
state without impacting any branches by switching back to a branch.

If you want to create a new branch to retain commits you create, you may
do so (now or later) by using -c with the switch command. Example:

  git switch -c <new-branch-name>

Or undo this operation with:

  git switch -

Turn off this advice by setting config variable advice.detachedHead to false

HEAD is now at d62d925823 [bpo-41710](https://bugs.python.org/issue41710): Add pytime_add() and pytime_mul() (GH-28642)
Switched to and reset branch 'main'

Could Not Find C:\buildbot.python.org\3.x.kloth-win64\build\Lib\*.pyc
The system cannot find the file specified.
Could Not Find C:\buildbot.python.org\3.x.kloth-win64\build\PCbuild\python*.zip

Could Not Find C:\buildbot.python.org\3.x.kloth-win64\build\Lib\*.pyc
The system cannot find the file specified.
Could Not Find C:\buildbot.python.org\3.x.kloth-win64\build\PCbuild\python*.zip

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot AMD64 Windows10 3.x has failed when building commit d62d925.

What do you need to do:

1. Don't panic.
2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
3. Go to the page of the buildbot that failed (https://buildbot.python.org/all/#builders/146/builds/1046) and take a look at the build logs.
4. Check if the failure is related to this commit (d62d925) or if it is a false positive.
5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/all/#builders/146/builds/1046

Summary of the results of the build (if available):

Click to see traceback logs

remote: Enumerating objects: 13, done.        
remote: Counting objects:   7% (1/13)        
remote: Counting objects:  15% (2/13)        
remote: Counting objects:  23% (3/13)        
remote: Counting objects:  30% (4/13)        
remote: Counting objects:  38% (5/13)        
remote: Counting objects:  46% (6/13)        
remote: Counting objects:  53% (7/13)        
remote: Counting objects:  61% (8/13)        
remote: Counting objects:  69% (9/13)        
remote: Counting objects:  76% (10/13)        
remote: Counting objects:  84% (11/13)        
remote: Counting objects:  92% (12/13)        
remote: Counting objects: 100% (13/13)        
remote: Counting objects: 100% (13/13), done.        
remote: Compressing objects: 100% (1/1)        
remote: Compressing objects: 100% (1/1), done.        
remote: Total 7 (delta 6), reused 6 (delta 6), pack-reused 0        
From https://github.com/python/cpython
 * branch            main       -> FETCH_HEAD
Note: checking out 'd62d925823b005c33b432e527562b573a3a89635'.

You are in 'detached HEAD' state. You can look around, make experimental
changes and commit them, and you can discard any commits you make in this
state without impacting any branches by performing another checkout.

If you want to create a new branch to retain commits you create, you may
do so (now or later) by using -b with the checkout command again. Example:

  git checkout -b <new-branch-name>

HEAD is now at d62d925... [bpo-41710](https://bugs.python.org/issue41710): Add pytime_add() and pytime_mul() (GH-28642)
Switched to and reset branch 'main'

Could Not Find D:\buildarea\3.x.bolen-windows10\build\Lib\*.pyc
The system cannot find the file specified.
Could Not Find D:\buildarea\3.x.bolen-windows10\build\PCbuild\python*.zip

Could Not Find D:\buildarea\3.x.bolen-windows10\build\Lib\*.pyc
The system cannot find the file specified.
Could Not Find D:\buildarea\3.x.bolen-windows10\build\PCbuild\python*.zip

[vstinner]

I don't understand how the two Windows jobs of the GitHub Action missed the build error. Anyway, I wrote PR #28644 to fix it.