Keep whitespace on updates (Python dependencies)

[jayfk]

Reference: pyupio/pyup#256

[rafaelpivato]

If you don't have the time to go through pyupio/pyup#256 this issue is about respecting an existing file format, like the one done by pip-compile, in a way that comments aligned to some virtual right column would stick to the same column, even when you change this:

somelibrary==1.9.2      # some comment
other==2.10.2           # some comment

...to something like this:

somelibrary==1.10.2     # some comment
other==2.10.2           # some comment

... and avoid something like this:

somelibrary==1.9.3 # some comment
other==2.10.2 # some comment