Helps #4796

Replacing

from distutils.command.build_ext import build_ext

with

from setuptools.command.build_ext import build_ext

At first, trying this fails with

  File "setuptools/command/build_ext.py", line 213, in build_extension
    if ext._needs_stub:
AttributeError: 'Extension' object has no attribute '_needs_stub'

_needs_stub is set in finalize_options() - https://github.com/pypa/setuptools/blob/145a78cdc1afb4ce0c0208ae86a00708eb79d27b/setuptools/command/build_ext.py#L162

So we need to have all Extensions created before running finalize_options. However, we use self.compiler to set some attributes of Extensions, and that isn't set at the time of finalize_options().

Instead, this PR suggests creating all Extensions at the very beginning, and then updating or removing them in build_extensions().