Lock dependencies in (test_)requirements.txt

[walro]

I guess we should lock to a major version at least.

https://pip.pypa.io/en/stable/user_guide/#requirements-files

[Chrizpy]

I tried running the tests with Python 3.9 and pytest 6.1.2. I also installed every package with pip 20.2.4 using the Makefile. When I ran the tests I got a total of 36 warnings and still green since 45 passed. To me, it looked like all warnings were deprecated related warnings, so I believe we could could lock things as they are now before things start to break. We could also fix the warnings and then do a lock file.

[walro]

A few of the warnings are emitted by us IIRC, we could remove those and then release next version as a major.