Normalizing path of requirement files

[ckleemann]

A requirements file could contain a relative link to another requirements file. If the link contains a .. it can not be handled by the GitLab api.

A short example:
The file requirements/production.txt contains the following:

-r ../subproject/other_requirements.txt

Pyup adds the file path requirements/../subproject/other_requirements.txt to the list of requirement files. The request to fetch this file from the GitLab api is then responded with an HTTP 400 Error.

By normalizing the file path before it is added this error is resolved. In our example the file path then would be subproject/other_requirements.txt.

[codecov]

Codecov Report

Merging #406 (82576c4) into master (17c2384) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #406   +/-   ##
=======================================
  Coverage   94.24%   94.25%           
=======================================
  Files          11       11           
  Lines        1164     1166    +2     
=======================================
+ Hits         1097     1099    +2     
  Misses         67       67           

Impacted Files	Coverage Δ
pyup/bot.py	97.89% <100.00%> (+0.01%)	⬆️

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