__signature__ attribute of a class is not writeable after importing pyside2

There was a recent report in CPython where importing pyside2 causes subsequent classes created after the import to have __signature__ as not writeable. This causes AttributeError when mocking objects where __signature__ is changed in the mocked object.  Since importing causes this problem I am filing this bug. Importing shiboken2 also causes same error where __signature__ could not be set. This has caused a regression in mock library and perhaps other programs that try to change __signature__. Any help in this is appreciated.

# sample reproducer

$ python
Python 3.8.0a4+ (heads/master:b1c3167c23, May 8 2019, 05:17:38)
[GCC 5.4.0 20160609] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> class Foo: pass
...
>>> Foo.__signature__ = 1
>>> import PySide2
>>> class Bar: pass
...
>>> Bar.__signature__ = 1
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
AttributeError: attribute '__signature__' of 'type' objects is not writable

 
Bug reports :

• https://bugs.python.org/issue36848
• https://github.com/testing-cabal/mock/issues/464
• https://github.com/pytest-dev/pytest-qt/issues/258