gh-104400: pygettext: Prepare to replace TokenEater with a NodeVisitor

[tomasr8]

Preparatory PR to keep the diff of #104402 as small as possible.

• Updates the module docstring
• Moves key_for inside the class
• Moves write_pot_file outside the class

cc @m-aciek

• Issue: pygettext: use an AST parser instead of a tokenizer #104400

[tomasr8]

Thank you both for such a quick review!

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot aarch64 RHEL8 LTO + PGO 3.x has failed when building commit e41ec8e.

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/#/builders/78/builds/8346) and take a look at the build logs.
4. Check if the failure is related to this commit (e41ec8e) 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/#/builders/78/builds/8346

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

==

Click to see traceback logs

Traceback (most recent call last):
  File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-aarch64.lto-pgo/build/Lib/threading.py", line 1054, in _bootstrap_inner
    self.run()
    ~~~~~~~~^^
  File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-aarch64.lto-pgo/build/Lib/threading.py", line 996, in run
    self._target(*self._args, **self._kwargs)
    ~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-aarch64.lto-pgo/build/Lib/test/test_interpreters/test_stress.py", line 47, in run
    interp = interpreters.create()
  File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-aarch64.lto-pgo/build/Lib/test/support/interpreters/__init__.py", line 76, in create
    id = _interpreters.create(reqrefs=True)
interpreters.InterpreterError: interpreter creation failed
k


Traceback (most recent call last):
  File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-aarch64.lto-pgo/build/Lib/threading.py", line 1054, in _bootstrap_inner
    self.run()
    ~~~~~~~~^^
  File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-aarch64.lto-pgo/build/Lib/threading.py", line 996, in run
    self._target(*self._args, **self._kwargs)
    ~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-aarch64.lto-pgo/build/Lib/test/test_interpreters/test_stress.py", line 30, in task
    interp = interpreters.create()
  File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-aarch64.lto-pgo/build/Lib/test/support/interpreters/__init__.py", line 76, in create
    id = _interpreters.create(reqrefs=True)
interpreters.InterpreterError: interpreter creation failed
k

[tomasr8]

The buildbot failure does not seem related