requests.Session().headers has a type of CaseInsensitiveDict; however, it is defined as being a MutableMapping[Text, Text] and so is missing a copy() method. Mypy incorrectly raises an error, error: "MutableMapping[str, str]" has no attribute "copy"

The definition for a CaseInsensitiveDict itself is also missing a copy() method

The copy() method is defined here
https://github.com/kennethreitz/requests/blob/4983a9bde39c6320aa4f3e34e50dac6e263dab6f/requests/structures.py#L80