As each test is executed (thus verifying the actual code too), it invokes
typecheck(). This runs mypy on the function that invoked it, parses the
reveal_locals() output, and optionally checks that each local's hinted
type is the same as its actual runtime time. This is optional because
some tests require more nuanced checks; cf test_pileup_iterator_column()
in which pileup()'s runtime type is a subclass of the hinted type.

Add test cases for previously merged PRs #1273, #1296, and #1313.

Also fix some pysam/libcbcf.pyi type hints that caused these tests
to fail for unrelated reasons.