Fixed mypy type errors in MLIR Python type stubs

This commit fixes or disables all errors reported by

    python3 -m mypy -p mlir --show-error-codes

Note that unhashable types cannot be currently expressed in a way compatible
with typeshed. See https://github.com/python/typeshed/issues/6243 for details.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D122790