drop python2 support

[Antoine Martin]

And take advantage of python3 features: ​https://datawhatnow.com/things-you-are-probably-not-using-in-python-3-but-should/

[Antoine Martin]

Started with a huge commit: r23802.

[Antoine Martin]

Updates:

• r23803 start removing gobject_compat module
• r23804 remove gobject from gobject_compat module
• r23805 remove gtk and gdk from gobject_compat module
• r23806 remove buffer and Queue indirect import
• r23807 don't use '_thread' module
• r23808 absolute imports are the default in py3k - no need to explicitly ask for them

[Antoine Martin]

Updates:

• r23809 + r23810 + r23811 + r23817 + r23817 remove gtk_util indirect calls, unused functions
• r23812 remove gio wrapper functions
• r23813 + r23816 + r23816 + r23819 remove unused code
• r23814 + r23815 fixups
• r23820 + r23821 ​PEP 3151
• r23822 ​PEP 0409
• r23823 remove unicode prefix
• r23824 assume subprocess.DEVNULL exists
• r23825 assume hashlib.algorithms_available exists

[Antoine Martin]

Things we can now take advantage of:

• ​importlib
• ​faulthandler.enable
• ​crypt module
• ​multiprocessing.Process: daemon flag?
• add ​sysconfig to xpra info
• ​enum
• ​type hints
• no need to retry EINTR syscalls: ​https://www.python.org/dev/peps/pep-0475/

[Antoine Martin]

faulthandler doesn't work?

[Antoine Martin]

Updates:

• r23836 fix handling of 'help' command line argument for audio codec options
• r23837 prefer matroskamux to webmmux if it is available
• r23838 explicitly set daemon flag so we can always create subprocesses from the proxy instance
• r23839 + r23841 we can assume os.cpu_count exists
• r23840 expose sysconfig via xpra info
• r23830 remove legacy .xpra location for log and socket dirs
• r23831 remove old GI_TYPELIB_PATH setup code for sound pipelines
• r23842 + r23843 + r23844 + r23845 + r23847: add more type hints (found this bug in the process: r23846)
• r23832 + r23834: simplify
• r23835 fixup
• r23833: support ​notify2 instead of pynotify

[Antoine Martin]

Updates:

• r23850 remove python2 moduleset
• r23851 remove python2 compat code

This will do for now, we can add more as we go along.

[Antoine Martin]

• r23877 no need to call ord on byte string characters

[Antoine Martin]

More: functions with dangerous default arguments: r24548 + r24549 + r24550.

[totaamwin32]

win32 env setup script: r24674

[migration script]

this ticket has been moved to: ​https://github.com/Xpra-org/xpra/issues/2290