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PyAPI: Use PyPreConfig & PyConfig for Python initialization

Use Python 3.8's API for setting the initial configuration.

This replaces a mix of our logic and direct calls to the Python API
and has no user visible changes.

Using the Python API makes the logic easier to follow and provides
utilities such as `PyConfig_SetBytesArgv`
that wasn't available in previous releases.

Note that this uses Python's utf8/wchar_t conversions,
which used to cause problems (see T31506).

Since `Py_UTF8Mode` was set, the systems locale isn't used for decoding,
allowing us to use Python's utility functions that call
`Py_DecodeLocale` internally.

Ref D10382