Remove Python 2 compatibility code

[kurtmckee]

As a side effect, the code coverage of unit tests is now 100%.

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[kurtmckee]

Signed the Google CLA. Thanks for reviewing this PR!

I'm using this package in a project I'm working on, and I'm hopeful to add a few additional tests after this merges (specifically, I'd like to use Hypothesis to test the round-tripping functionality, as it is excellent at finding round-trip issues).

[suminb]

Looks good to me. Appreciate it!

[suminb]

I'll merge and publish this as v0.6.0 as soon as possible. Thanks for your PR!

[kurtmckee]

You're welcome! Thanks for your work on base62!