[3.7] bpo-34247: Fix Python 3.7 initialization (#8659)

[3.7] bpo-34247: Fix Python 3.7 initialization (#8659)

* -X dev: it is now possible to override the memory allocator using
  PYTHONMALLOC even if the developer mode is enabled.
* Add _Py_InitializeFromConfig()
* Add _Py_Initialize_ReadEnvVars() to set global configuration
  variables from environment variables
* Fix the code to initialize Python: Py_Initialize() now also reads
  environment variables
* _Py_InitializeCore() can now be called twice: the second call
  only replaces the configuration.
* Write unit tests on Py_Initialize() and the different ways to
  configure Python
* The isolated mode now always sets Py_IgnoreEnvironmentFlag and
  Py_NoUserSiteDirectory to 1.
* pymain_read_conf() now saves/restores the configuration
  if the encoding changed