* python: adjust basic typing information
This is an initial pass to fix and adjust the typing information for the
generated client. This is enough to have mypy runnning without complains
on all the (modern) generated clients (Pydantic v1 code is not checked
for instance)
mypy is also now run directly in the CI, so further changes will also be
checked and thus, will need to be compliant with good typing
information.
Note that this doesn't *fully* type all the code: mypy is not run in
"strict" mode and there are still many functions/methods/attributes
which are still not fully typed, but it's a first good step in that
direction.
* ApiResponse's raw_data can't be None
* Fix indentation
* Revert test changes
* run mypy on tests/ directory
* don't forcefully convert the client response headers to dict
* override petstore ApiResponse model
* adjust type of 'any/one_of_schemas' fields
