Well, after more discussions with kparal, we are still unsure about the "right" way to tackle this.
Our current call would be:
1) sync requirements.txt versions with fedora (mostly done)
2) allow --system-site-packages in the test_env
3) do `pip install -r requirements.txt` (with possible flags to enforce versions) to the makefile virtualenv creation step
4) add info to readme, that testing needs installation of packages from pypi, and that some of them need compilation
4-1) put together a list of packages that need to be installed (the python-foobar kind, not -devel + gcc) to the system, in order to "skip" the stuff that needs to be compiled

Sounds reasonable, Kamil? Others?

I went back and forth on this. I thought it would be a really simple change, and as usual, it seems more pain than gain. So, I went forward with this:
1. add tox.ini to projects to allow simple test suite execution with `pytest` (non-controversial)
2. configure tox.ini to print out test coverage (non-controversial)
3. remove --system-site-packages from all places (readme, makefile) for those projects, that can be *fully* installed from pypi *without any compilation* (hopefully non-controversial).
4. keep (or add) --system-site-packages to readme/makefile for the remaining projects, and add readme info how to deal with pypi compilation or local rpm installation

What Josef mentioned is that he wouldn't try to replicate a perfect environment directly on dev machine, because that's  a lot of work. Instead, use the current non-perfect environment on dev machines (which should be fine most of the time anyway) and have a separate CI service (hopefully in the future) with more strict environment configuration. I guess that's the most practical solution.

We might even want to reopen the question how to version deps in requirements.txt vs spec file, but I'd keep that for a separate thread, if needed.

My current patches for resultsdb projects are these:
https://phab.qa.fedoraproject.org/D1114
https://phab.qa.fedoraproject.org/D1116
https://phab.qa.fedoraproject.org/D1117