Fix test_embed.test_pre_initialization_sys_options() env vars

[vstinner]

test_pre_initialization_sys_options() of test_embed now removes
PYTHON* environment variables like PYTHONWARNINGS.

[vstinner]

For the record, this PR fix the following bug:

$ PYTHONWARNINGS=error ./python -m test test_embed -m test_pre_initialization_sys_options -v
== CPython 3.9.0a0 (heads/master-dirty:00f6493084, Jun 17 2019, 21:50:32) [GCC 9.1.1 20190503 (Red Hat 9.1.1-1)]
== Linux-5.1.6-300.fc30.x86_64-x86_64-with-glibc2.29 little-endian
== cwd: /home/vstinner/prog/python/master/build/test_python_32058
== CPU count: 8
== encodings: locale=UTF-8, FS=utf-8
Run tests sequentially
0:00:00 load avg: 0.79 [1/1] test_embed
test_pre_initialization_sys_options (test.test_embed.EmbeddingTests) ... FAIL

======================================================================
FAIL: test_pre_initialization_sys_options (test.test_embed.EmbeddingTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/vstinner/prog/python/master/Lib/test/test_embed.py", line 243, in test_pre_initialization_sys_options
    self.assertIn(expected_output, out)
AssertionError: "sys.warnoptions: ['once', 'module', 'default']\nsys._xoptions: {'not_an_option': '1', 'also_not_an_option': '2'}\nwarnings.filters[:3]: ['default', 'module', 'once']\n" not found in "Checking PySys_AddWarnOption\nChecking PySys_ResetWarnOptions\nChecking PySys_AddWarnOption linked list\nChecking PySys_AddXOption\nInitializing interpreter\nCheck sys module contents\nFinalizing interpreter\nsys.warnoptions: ['error', 'once', 'module', 'default']\nsys._xoptions: {'not_an_option': '1', 'also_not_an_option': '2'}\nwarnings.filters[:3]: ['default', 'module', 'once']\n"

----------------------------------------------------------------------

Ran 1 test in 0.069s

FAILED (failures=1)
test test_embed failed
test_embed failed

== Tests result: FAILURE ==

1 test failed:
    test_embed

Total duration: 246 ms
Tests result: FAILURE

[miss-islington]

Thanks @vstinner for the PR 🌮🎉.. I'm working now to backport this PR to: 3.8.
🐍🍒⛏🤖

[bedevere-bot]

GH-14176 is a backport of this pull request to the 3.8 branch.