Release the GIL when reading from BytesIO objects. (#291)

* Add special case to release the GIL when reading from a BytesIO object.

* Fix tests.

* Disable flakey test on underpowered machines.

* Decrease test flakiness.

* Disable flaky test on CI.

* Skip on CI properly.

* Ensure that we can't read past the end of a BytesIO buffer.

* Use one of virtual or override.

* Apply suggestions from code review

Co-authored-by: David Rubinstein <drubinstein@users.noreply.github.com>

* Simplify read math for PythonMemoryViewInputStream.

* Fix type annotations in docs.

* Appease the linter.

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Co-authored-by: David Rubinstein <drubinstein@users.noreply.github.com>