Resolve python3 dependency versions

[vkuznet]

Impact of the bug
The python dependencies listed requirements_py3.txt should be properly fixed to find all working versions.

Describe the bug
running python3 -m pip install -r requirements_py3.txt yields the following output

| The conflict is caused by:
|     rucio-clients 1.25.5 depends on requests<=2.25.0 and >=2.20.0
|     sphinx 4.0.3 depends on requests>=2.5.0
|     cmscouchapp 1.3.2 depends on requests==2.25.1

How to reproduce it

python3 -m pip install -r requirements_py3.txt

Expected behavior
I expect that all listed dependencies should be properly resolved. At the moment I have no clue which versions of listed dependencies are required for WMCore services and it will be best if experts provide correct list of them.

Additional context and error message