Handle PEP-496 Environment Markers

[nanonyme]

requirements-parser doesn't handle them internally so we need to
extract them from original line. Not handling these results in
unintended things being regarded as dependencies

[nanonyme]

This isn't super-secure since it evaluates the trailer as code but it should showcase the problem and be useful as workaround if you review requirements.txt beforehand.

[TingPing]

Maybe prompt before doing this?

[nanonyme]

@TingPing it would not work with automatic updates.

[nanonyme]

Technically right way probably is to use ast module to parse it and interpret the result manually making sure we bail out if something weird is noticed.