Instead of raising an exception, warn when a pytest-mock mock is used as
a context manager. This lets pytest-mock mock objects that are used as
context managers, like threading.Lock, while still letting users know
that mocks returned from pytest-mock do not need their __enter__ method
called to take effect.

Create a PytestMockWarning class so that the warning is easy to
pinpoint and handle.

Test that warnings are issued when mocks are used as context manager.
Verify that the warnings have the expected message and that they point
to the correct code.

Use a stacklevel=5 for Python 3.8 and up so that the emitted warning
points to the correct line of code. At stacklevel=4, it was incorrectly
pointing to code in the standard library.

Make the warning message when a mock is used as a context manager more
detailed. In the message, describe the use case to avoid and when the
warning can be safely ignored.

Create patch.context_manager method that allows the patched object to be
used as a context manager without issuing a warning on __enter__.
Otherwise, the method is functionally equivalent to patch.object.