Fix unit tests, lint, drop Python 2 support

[FrancoisHuet]

Drop EOL Python 2 support. Resolve unit test discrepancies. Update unit tests to pass in Travis across all supported Py. Linting.

[MartinThoma]

Hey!

As camelot is dead, we try to build a maintained fork at pypdf_table_extraction.

Do you want to open the PR against that branch so that we can merge your improvement?

[FrancoisHuet]

Martin - unfortunately I'm unlikely to have the bandwidth to do this in the near future. I can try to squeeze this in if there's demand on this PR and likely quick turnaround on merging it.

[bosd]

Thanks for your contribution. This has been merged in adifferent pr.