bpo-30565: Add PYTHONCOERCECLOCALE=warn runtime flag (GH-2260)

- removes PY_WARN_ON_C_LOCALE build time flag
- locale coercion and compatibility warnings are now always compiled
  in, but are off by default
- adds PYTHONCOERCECLOCALE=warn runtime option to aid in
  debugging potentially locale related compatibility problems

Due to not-yet-resolved test failures on *BSD systems (including
Mac OS X), this also temporarily disables UTF-8 as a locale coercion
target, and skips testing the interpreter's behavior in the POSIX locale.

Doc/using/cmdline.rst

@@ -744,6 +744,11 @@ conflict.
    :data:`sys.stdin` and :data:`sys.stdout` to ``surrogateescape``. This
    behavior can be overridden using :envvar:`PYTHONIOENCODING` as usual.
 
+   For debugging purposes, setting ``PYTHONCOERCECLOCALE=warn`` will cause
+   Python to emit warning messages on ``stderr`` if either the locale coercion
+   activates, or else if a locale that *would* have triggered coercion is
+   still active when the Python runtime is initialized.
+
    Availability: \*nix
 
    .. versionadded:: 3.7