Add Github CI workflow

[jku]

Description of the changes being introduced by the pull request:

Because of recent issues with travis I ended up polishing my Github CI commit a bit:

• tests enabled on linux and mac
• windows is disabled due to gpg hanging and a storage test failure: these need to be investigated
• The no-gpg cases are now handled using the new environment variable
• No coveralls support

[jku]

The Windows gpg failures look like this

TimeoutExpired: Command '['gpg', '--detach-sign', '--digest-algo', 'SHA256', '--homedir', 'c:/users/runner~1/appdata/local/temp/tmprxgwki/rsa']' timed out after 3 seconds

And the storage test failure:

======================================================================
FAIL: test_exceptions (tests.test_storage.TestStorage)
----------------------------------------------------------------------
Traceback (most recent call last):
File "D:\a\securesystemslib\securesystemslib\tests\test_storage.py", line 66, in test_exceptions
    self.storage_backend.create_folder, '/none/existent/path')
 AssertionError: StorageError not raised

[coveralls]

Coverage remained the same at 98.531% when pulling 3c7f65d on jku:gh-actions into dff4425 on secure-systems-lab:master.

[lukpueh]

Thanks for adding the workflow file, @jku! It looks quite good to me. We should ticketize the remaining tasks i.e.:

• reinstate coveralls
• enable windows builds

I can do that if you want.

I also suggest to remove .travis.yml and all traces to it (badge in readme, manifest.in directive, comment in tox.ini). Can you do that? Oh and maybe add a new GH action badge to the readme?

[lukpueh]

Good catch! Interesting how tox successfully builds a py39purepy27 env. :)

[jku]

I think I finally managed to get the badge right... but we'll see once there are results for master. The PR should good to go. I can file the two issues after lunch (if you haven't at that point).

[lukpueh]

@jku, I added the two tickets #325 and #326, and also took the liberty to push a commit here that removes the coveralls badge (until we fix #325).

[jku]

Damn it, almost got the workflow link correct on the badge...

[lukpueh]

Damn it, almost got the workflow link correct on the badge...

Can you send a fix?