bpo-42260: Compute the path config in the main init

[vstinner]

The path configuration is now computed in the "main" initialization.
The core initialization no longer computes it.

• Add _PyConfig_Read() function to read the configuration without
  computing the path configuration.

• pyinit_core() no longer computes the path configuration: it is now
  computed by init_interp_main().

• The path configuration output members of PyConfig are now optional:

  • executable
  • base_executable
  • prefix
  • base_prefix
  • exec_prefix
  • base_exec_prefix

• _PySys_UpdateConfig() now skips NULL strings in PyConfig.

• _testembed: Rename test_set_config() to test_init_set_config() for
  consistency with other tests.

https://bugs.python.org/issue42260

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot PPC64LE RHEL8 LTO + PGO 3.x has failed when building commit 9e1b828.

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/458/builds/729) and take a look at the build logs.
4. Check if the failure is related to this commit (9e1b828) 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/458/builds/729

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

== Tests result: ENV CHANGED ==

410 tests OK.

10 slowest tests:

• test_concurrent_futures: 2 min 45 sec
• test_tokenize: 1 min 36 sec
• test_multiprocessing_spawn: 1 min 33 sec
• test_unparse: 1 min 6 sec
• test_multiprocessing_forkserver: 1 min 2 sec
• test_lib2to3: 1 min 1 sec
• test_asyncio: 58.2 sec
• test_multiprocessing_fork: 57.4 sec
• test_signal: 49.3 sec
• test_io: 48.9 sec

1 test altered the execution environment:
test_asyncio

14 tests skipped:
test_devpoll test_gdb test_ioctl test_kqueue test_msilib
test_ossaudiodev test_startfile test_tix test_tk test_ttk_guionly
test_winconsoleio test_winreg test_winsound test_zipfile64

Total duration: 5 min 1 sec

Click to see traceback logs

Traceback (most recent call last):
  File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-ppc64le.lto-pgo/build/Lib/asyncio/sslproto.py", line 321, in __del__
    self.close()
  File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-ppc64le.lto-pgo/build/Lib/asyncio/sslproto.py", line 316, in close
    self._ssl_protocol._start_shutdown()
  File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-ppc64le.lto-pgo/build/Lib/asyncio/sslproto.py", line 590, in _start_shutdown
    self._abort()
  File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-ppc64le.lto-pgo/build/Lib/asyncio/sslproto.py", line 731, in _abort
    self._transport.abort()
  File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-ppc64le.lto-pgo/build/Lib/asyncio/selector_events.py", line 680, in abort
    self._force_close(None)
  File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-ppc64le.lto-pgo/build/Lib/asyncio/selector_events.py", line 731, in _force_close
    self._loop.call_soon(self._call_connection_lost, exc)
  File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-ppc64le.lto-pgo/build/Lib/asyncio/base_events.py", line 746, in call_soon
    self._check_closed()
  File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-ppc64le.lto-pgo/build/Lib/asyncio/base_events.py", line 510, in _check_closed
    raise RuntimeError('Event loop is closed')
RuntimeError: Event loop is closed