Update setuptools.txt

[geryogam]

This PR:

• uses the long option --upgrade consistently;
• uses the built-in exec function instead of the Python 2-only execfile;
• fixes a command typo installing setuptools and wheel twice instead of twine;
• removes inconsistent --user options in some pip install commands;
• calls pip install from the PATH consistently instead of from the module namespace (python -m pip install) as we are not upgrading pip itself on Windows anywhere;
• fixes a URI.

[jaraco]

Thank you very much for this gift.