Consider dropping support of Python 3.6

[vkarak]

We need to identify the places in the code where we have workarounds needed for Python 3.6. If we move to Python 3.7, we are also guaranteed that dictionaries are ordered, so we could drop the use of OrderedDict.

[haampie]

Is there a need to drop Python 3.6? Even if it's EOL, it's still the system default on CentOS 7 (EOL Jun 2024) and Ubuntu 18.04 (EOL Apr 2028)

[vkarak]

Currently, there is no big need in doing so from the point of view of maintaining the code. It's just very few places that we need to maintain 3.6-specific code.

[vkarak]

Decided not to drop support.