The following script runs fine on Python 3.7.1 but not on master (f5107dfd42).

import logging

class Foo(logging.Formatter):
    def __init__(self):
        super().__init__(self)

Foo()

The output is:

Traceback (most recent call last):
  File "t.py", line 9, in <module>
    Foo()
  File "t.py", line 6, in __init__
    super().__init__(self)
  File "/usr/lib/python3.8/logging/__init__.py", line 589, in __init__
    self._style.validate()
  File "/usr/lib/python3.8/logging/__init__.py", line 441, in validate
    if not self.validation_pattern.search(self._fmt):
TypeError: expected string or bytes-like object

Most likely there's something wrong in the newly-added validation step (issue34844).

/cc the primary reviewer of the aforementioned patch.