Refactor gettext code

[declension]

• Use pathlib paths everywhere. It's neater now and we get better checking
• Add lots of typing, mypy does some work for us now
• Use f-strings for great win
• Fix some inappropriate test naming...
• Improve some old / incorrect docstrings
• Add some tests around setup.py PO commands (this broke them initially, but no tests broke)

Fixes #3736

[X-dark]

Seems I cannot build with that change. Is this maybe because of a missing dep?

Traceback (most recent call last):
  File "/build/quodlibet-git/src/quodlibet-git/setup.py", line 131, in <module>
    main()
  File "/build/quodlibet-git/src/quodlibet-git/setup.py", line 127, in main
    setup(**setup_kwargs)
  File "/usr/lib/python3.9/distutils/core.py", line 148, in setup
    dist.run_commands()
  File "/usr/lib/python3.9/distutils/dist.py", line 966, in run_commands
    self.run_command(cmd)
  File "/usr/lib/python3.9/distutils/dist.py", line 985, in run_command
    cmd_obj.run()
  File "/usr/lib/python3.9/distutils/command/build.py", line 135, in run
    self.run_command(cmd_name)
  File "/usr/lib/python3.9/distutils/cmd.py", line 313, in run_command
    self.distribution.run_command(command)
  File "/usr/lib/python3.9/distutils/dist.py", line 985, in run_command
    cmd_obj.run()
  File "/build/quodlibet-git/src/quodlibet-git/gdist/po.py", line 260, in run
    basepath = self.build_base / "share" / "locale"
TypeError: unsupported operand type(s) for /: 'str' and 'str'

[declension]

Thanks @X-dark, my bad; guess I missed those. Moved to #3743

[lazka]

this made paths in quodlibet.pot absolute somehow

[lazka]

fixed in #3798