Refactoring to support overlapping dependencies

[dsimidzija]

Hi there!

Recently I forked marshmallow to try to get some performance improvements, so I tried to run the benchmarks on my machine from the latest master in this repo. Several things stood out to me:

1. The docker image should be pinned to python 3.9 instead of going to the latest 3.x because there will always be some lag between the release of python and various libraries. This is the cause of the current master being broken/unrunnable, unless the image is pinned.

2. I would like to create a PR which would attempt to refactor this repo in order to separate each library into a separate virtualenv, so we can avoid things like Test against marshmallow 3 #12 where the benchmarks are actually misrepresenting latest marshmallow. But before I attempt this, I would like to know if you'd be willing to accept an PR like that, as it would probably take some valuable spare time to do it, and it would be a major rewrite. (Would probably require a completely new result collection method.)

3. Last but not least, I would like to improve the tests a bit. Currently - provided I'm reading the code correctly - the difference between test_one and test_many is just the format, but I believe that the test for test_many would be more realistic if we attempt to serialize 1k objects, or 10k objects instead of just two. Let me know if you're open to that idea or not.