Remove explicit "hypothesis" pytest markers

[mattsb42-aws]

Starting to test this, it looks like this error first appears in 3.64.0

https://travis-ci.org/awslabs/base64io-python/builds/397145142

[mattsb42-aws]

HypothesisWorks/hypothesis#1362

[mattsb42-aws]

Per guidance from hypothesis, it looks like the root cause of this is that we add a "hypothesis" pytest marker to tests that use hypothesis, but hypothesis already adds this marker (currently undocumented). Hypothesis is following up with pytest about the issue with the same marker being applied multiple times, but our action here is just to remove our explicit "hypothesis" markers.

[mattsb42-aws]

Looks like pytest fixed the underlying issue with re-applying markers. We should still remove the hypothesis markers though, to avoid breaking on previous pytest versions.