unsplit the wrapper mixins

[davidism]

• base_request.py and base_response.py were renamed to request.py and response.py. BaseRequest became Request, and BaseResponse became Response. All mixin code was moved into the classes.
• All mixins show a deprecation warning in __init__.
• BaseRequest and BaseResponse can still be used in isinstance and issubclass, they will show a deprecation warning and actually check against Request and Response.
• PlainRequest is deprecated in favor of passing shallow=True, since that's just as effective at preventing reading the input stream.
• Rewrote the docstrings for Request and Response.
• ETagResponseMixin provided an overridden freeze method, but it wasn't active on the Response class due to the inheritance order. It adds an ETag header if one is not already set. I kept/added this behavior during the merge, since it made about as much sense as setting the Content-Length header. I also deprecated the no_etag parameter (that wasn't visible in Response either), since I didn't see a clear reason to allow choosing this during freeze and didn't want to open up the door to more such parameters if freeze is updated in the future.
• Merged JSONMixin and adapted it to be specific to request vs response behavior. Removed caching for response.get_json (Cache Param For get_json() Leading to Contradictory Behavior #1698) as it caused issues if the data changed. Removed special exception handling for response. For some reason the mixin was written to use json_module, even though only loads is called. I left it as-is because it avoids having to wrap staticmethod(json.loads), and having dumps as well might be useful in the future. The request and response now have json, get_json, and related methods by default.
• Using git blame, git doesn't see that response.py was actually base_response.py, and so the history doesn't look very interesting. Using git blame -M -C will tell it to figure out that the file moved instead, so it correctly sees the history from base_response.py and the mixin modules (and wrappers.py further back in history). PyCharm can also do this by selecting the "Detect movements across files" option when right clicking in the annotation sidebar.

• fixes unsplit the wrapper mixins #1963
• fixes Cache Param For get_json() Leading to Contradictory Behavior #1698

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.