* Revert "bpo-36097: Use only public C-API in the_xxsubinterpreters module (adding as necessary). (#12003)"

This reverts commit bcfa450f210074e16feb761ae5b3e966a2532fcf.

* Revert "bpo-33608: Simplify ceval's DISPATCH by hoisting eval_breaker ahead of time. (gh-12062)"

This reverts commit bda918bf65a88560ec453aaba0758a9c0d49b449.

* Revert "bpo-33608: Use _Py_AddPendingCall() in _PyCrossInterpreterData_Release(). (gh-12024)"

This reverts commit b05b711a2cef6c6c381e01069dedac372e0b9fb2.

* Revert "bpo-33608: Factor out a private, per-interpreter _Py_AddPendingCall(). (GH-11617)"

This reverts commit ef4ac967e2f3a9a18330cc6abe14adb4bc3d0465.

Add TEST_EXTENSIONS constant to setup.py to allow to not build test
extensions like _testcapi.

Changes:

* Add add_ldflags_cppflags() subfunction
* Rename add_compiler_directories() to configure_compiler().
* Remove unused COMPILED_WITH_PYDEBUG constant.
* Use self.add() rather than accessing directly self.extensions.
* Remove module_enabled() function: check differently if curses
  extension is built or not.

* Move set_compiler_flags() calls and concurrent.future hack from
  module top-level to main()
* Remove unused variables 'macros' and 'libraries' from
  detect_multiprocessing().
* Move SUMMARY and CLASSIFIERS constants at the top, move
  set_compiler_flags() function below these constants.
* Add some empty new lines to respect PEP 8.

* Split PyBuildExt.detect_modules() huge function into subfunctions.
* Move curses, hashlib and some other code to reorganize the code.
* detect_tkinter() now returns False if the extension is missing.
* Add PyBuildExt.config_h_vars attribute

* Add PyBuildExt.srcdir atribute in setup.py: the source directory is
  now always absolute.
* Add PyBuildExt.inc_dirs and PyBuildExt.lib_dirs attributes:
  replace 'inc_dirs' and 'lib_dirs' local variables of
  detect_modules().
* Replace "from distutils.errors import *"
  with "from distutils.errors import CCompilerError, DistutilsError"
  to be able to use static analyzers like pyflakes
* Reorder imports.

* Add PyBuildExt.add() which adds the extension directly to
  self.extensions, rather than using a temporary 'exts' local
  variable in detect_modules() and then add 'exts' to self.extensions
* Convert 'missing' local variable from detect_modules()
  into PyBuildExt.missing attribute
* _detect_openssl(), _decimal_ext() and _detect_nis() now call
  directly self.add(), rather than returning an extension
  (or None if not found).
* Rename _decimal_ext() to _detect_decimal() for consistency with
  other methods.

Fix setup.py on macOS: only add /usr/include/ffi to include
directories of _ctypes, not for all extensions.

* Rename globals to upper case to better distinguish if a variable is
  global or local:

  * Rename cross_compiling to CROSS_COMPILING
  * Rename host_platform to HOST_PLATFORM
  * Rename disabled_module_list to DISABLED_MODULE_LIST

* Add MS_WINDOWS, CYGWIN and MACOS constants.
* Use booleans: replace "return 0" with "return False"
  and replace "return 1" with "return True"