Based on tests from #4878

Exactly which locale requests will end up giving
you the "C" locale is actually platform dependent.

A blank locale and "POSIX" will translate to "C"
on most Linux distros, but may not do so on other platforms, so this adjusts the way the tests are structured to better account for that.

This is an initial step towards fixing the current
test failure on Cygwin (hence the issue reference) 

As a result of 92a3c6f4, the compiler complains:

Python/import.c:2311:21: warning: comparison of integers of different signs: 'long' and 'unsigned long' [-Wsign-compare]
    if ((i + n + 1) <= PY_SSIZE_T_MAX / sizeof(struct _inittab)) {
         ~~~~~~~~~  ^  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This overflow is extremely unlikely to happen, but let's avoid undefined
behavior anyway.

bpo-29240, bpo-32030: If the encoding change (C locale coerced or
UTF-8 Mode changed), Py_Main() now reads again the configuration with
the new encoding.

Changes:

* Add _Py_UnixMain() called by main().
* Rename pymain_free_pymain() to pymain_clear_pymain(), it can now be
  called multipled times.
* Rename pymain_parse_cmdline_envvars() to pymain_read_conf().
* Py_Main() now clears orig_argc and orig_argv at exit.
* Remove argv_copy2, Py_Main() doesn't modify argv anymore. There is
  no need anymore to get two copies of the wchar_t** argv.
* _PyCoreConfig: add coerce_c_locale and coerce_c_locale_warn.
* Py_UTF8Mode is now initialized to -1.
* Locale coercion (PEP 538) now respects -I and -E options.

Don't define decode_locale() nor encode_locale() on macOS or Android.

Link to the Format Specification Mini Language section from f-strings' documentation.

Run the child process with -E option to ignore the PYTHONWARNINGS
environment variable.

Add types.ClassMethodDescriptorType for unbound class methods.

It no longer spends much time doing complex calculations and no
longer consumes much memory for creating large constants that will
be dropped later.

This fixes also bpo-21074.

This fixes also bpo-22091.

* Implement functionality
* Add documentation

* Add new fields to _PyMainInterpreterConfig:

  * executable
  * prefix
  * base_prefix
  * exec_prefix
  * base_exec_prefix

* _PySys_EndInit() now sets sys attributes from
  _PyMainInterpreterConfig

Each interpreter now has its core_config and main_config copy:

* Add _PyCoreConfig_Copy() and _PyMainInterpreterConfig_Copy()
* Move _PyCoreConfig_Read(), _PyCoreConfig_Clear() and
  _PyMainInterpreterConfig_Clear() from Python/pylifecycle.c to
  Modules/main.c
* Fix _Py_InitializeEx_Private(): call _PyCoreConfig_ReadEnv() before
  _Py_InitializeCore()

bpo-32329, bpo-32030:

* The -R option now turns on hash randomization when the
  PYTHONHASHSEED environment variable is set to 0 Previously, the
  option was ignored.
* sys.flags.hash_randomization is now properly set to 0 when hash
  randomization is turned off by PYTHONHASHSEED=0.
* _PyCoreConfig_ReadEnv() now reads the PYTHONHASHSEED environment
  variable. _Py_HashRandomization_Init() now only apply the
  configuration, it doesn't read PYTHONHASHSEED anymore.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

This part of the PEP implementation adds support for
__mro_entries__ and __class_getitem__ by updating
__build_class__ and PyObject_GetItem.

Conversions like `not a is b -> a is not b` are implemented
in AST optimizer in previous commit (7ea143ae).
So this commit removes them from peephole optimizer.

Add warnoptions and xoptions fields to _PyMainInterpreterConfig.

* Fix multiple typos in code comments

* Add spacing in comments (test_logging.py, test_math.py)

* Fix spaces at the beginning of comments in test_logging.py

Implement PEP 562: module __getattr__ and __dir__.
The implementation simply updates module_getattro and
module_dir.

CRT v142 is binary compatible with CRT v140.

_PyMainInterpreterConfig now contains Python objects, whereas
_PyCoreConfig contains wchar_t* strings.

Core config:

* Rename _PyMainInterpreterConfig_ReadEnv() to _PyCoreConfig_ReadEnv()
* Move 3 strings from _PyMainInterpreterConfig to _PyCoreConfig:
  module_search_path_env, home, program_name.
* Add _PyCoreConfig_Clear()
* _PyPathConfig_Calculate() now takes core config rather than main
  config
* _PyMainInterpreterConfig_Read() now requires also a core config

Main config:

* Add _PyMainInterpreterConfig.module_search_path: sys.path list
* Add _PyMainInterpreterConfig.argv: sys.argv list
* _PyMainInterpreterConfig_Read() now computes module_search_path

Changes:

* Split _PySys_SetArgvWithError() into subfunctions for Py_Main():

  * Create the Python list object
  * Set sys.argv to the list
  * Compute argv0
  * Prepend argv0 to sys.path

* Add _PyPathConfig_ComputeArgv0()
* Remove _PySys_SetArgvWithError()
* Py_Main() now splits the code to compute sys.argv/path0 and the
  code to update the sys module: add pymain_compute_argv()
  subfunction.

asyncio.get_event_loop(), and, subsequently asyncio._get_running_loop()
are one of the most frequently executed functions in asyncio.  They also
can't be sped up by third-party event loops like uvloop.

When implemented in C they become 4x faster.

bpo-29240, bpo-32030:

* Rename pymain_set_argv() to pymain_set_sys_argv()
* pymain_set_sys_argv() now creates of copy of argv and modify the
  copy, rather than modifying pymain->argv
* Call pymain_set_sys_argv() earlier: before pymain_run_python(), but
  after pymain_get_importer().
* Add _PySys_SetArgvWithError() to handle errors