- Some tests used unittest.mock from the standard library
 - Some tests used mock from PyPI
 - Some tests tried to import unittest.mock with a fallback to mock
   (the import never fails on Python 3.7+, older Pythons are not supported)

The upload and register commands were deprecated over a year ago, in
July 2018 (PR GH-1410, discussed in issue GH-1381). It is time to
actively remove them in favor of twine.

This comment is not used anywhere and `platform.dist()` is deprecated.

See CPython PR #10414: https://github.com/python/cpython/pull/10414
and bpo-35186: https://bugs.python.org/issue35186

The `upload.show_response` feature was not added until Python 3. Rather
than backport it, it is now enabled only if supported.

This also adds a "smoke test" for the feature.

This fixes uploads when bdist_rpm or bdist_dumb are the command, both of
which insert a comment about what platform they are built for.

This fixes an issue where `distutils.spawn.spawn` was not available in
the ported upload_file, which is only used when signing the data.

This also adds a test that the gpg signature command is invoked and
included in the uploaded data.