All extra names must be converted to lower case. use pkg_resources.sa…

[GabrielC101]

Fixes pypa/pip#4617. Supposedly.

[benoit-pierre]

This patch is mangling the environment markers.

[GabrielC101]

I'll have to come back to this later.

[GabrielC101]

Note: setuptools makes the extras safe when parsing the Requirement object args. https://github.com/pypa/setuptools/blob/master/pkg_resources/__init__.py#L3006

[GabrielC101]

setuptools.pkg_resources.evaluate_marker at https://github.com/pypa/setuptools/blob/master/pkg_resources/__init__.py#L1449 claims to determine if a string is a valid marker. Yet the string ':python_version=="2.7"' is considered invalid.

Is evaluate_marker reliable? Ultimately the decision is made by the pyparsing grammer. I'm having difficulty deciphering it.

[benoit-pierre]

':python_version=="2.7"' is not a valid marker, 'python_version=="2.7"' is.

[GabrielC101]

This commit uses the same process used in _check_extra to separate an extra name from the environment markers. It then uses the safe_extras method to convert the extra name before recreating the section.

This ensures setuptools views extra names as case insensitive.