Fix parsing requirement version when inline comment exists

[pavdmyt]

Hi,

I'm using pypiup with pip-tools in my projects. pip-tools compiles requirements.txt and automatically fills dependencies as inline comments:

#
# This file is autogenerated by pip-compile
# To update, run:
#
#    pip-compile --output-file requirements-dev.txt requirements-dev.in
#
appnope==0.1.0            # via ipython
astroid==1.5.3            # via pylint
certifi==2017.4.17        # via requests
chardet==3.0.3            # via requests
click==6.7                # via pip-tools, pypiup
decorator==4.0.11         # via ipython, traitlets
first==2.0.1              # via pip-tools
flake8==3.3.0
idna==2.5                 # via requests

Unfortunately pypiup==0.0.2 fails to compare such kind of lines with the following result:

✗ appnope
  Could not compare. Invalid semver, From 0.1.0            # via ipython to 0.1.0.

✗ astroid
  Could not compare. Invalid semver, From 1.5.3            # via pylint to 1.5.3.

✗ certifi
  Could not compare. Invalid semver, From 2017.4.17        # via requests to 2017.4.17.

✗ chardet
  Could not compare. Invalid semver, From 3.0.3            # via requests to 3.0.4.

✗ click
  Could not compare. Invalid semver, From 6.7                # via pip-tools, pypiup to 6.7.

✗ decorator
  Could not compare. Invalid semver, From 4.0.11         # via ipython, traitlets to 4.0.11.

✗ first
  Could not compare. Invalid semver, From 2.0.1              # via pip-tools to 2.0.1.

✓ flake8
  Up to date, 3.3.0.

✗ idna
  Could not compare. Invalid semver, From 2.5                 # via requests to 2.5.

This PR fixes requirement version parsing making it possible to combine pypiup with pip-tools in the projects.

[codecov-io]

Codecov Report

Merging #13 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #13   +/-   ##
=======================================
  Coverage   99.18%   99.18%           
=======================================
  Files           4        4           
  Lines         123      123           
=======================================
  Hits          122      122           
  Misses          1        1

Impacted Files	Coverage Δ
pypiup/requirement.py	100% <100%> (ø)	⬆️

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[pavdmyt]

@manosim Any chance we can get this merged, or a similar PR?

[manosim]

Hello @pavdmyt! Both PRs look great thank you very much for sending them over! Can you add a test for this case as well (add a requirement with a comment in requirements-tests.txt)?

[pavdmyt]

@manosim done.

[pavdmyt]

Hey @manosim any updates, is it now Ok for merge?

[manosim]

Hello @pavdmyt! Sorry was on holidays for the past three weeks! Just tested this one and it looks good! Thank you for opening this PR - really appreciate it! 😄