Drop pybind11 < 2.3 support

[mosra]

Currently the python documentation code supports pybind11 2.2 and 2.3 because 2.3 was released a short while ago and not all systems have the new package yet. However, there's a lot of significant changes in between those versions that dropping support for 2.2 at some point (~six months from now?) will improve maintainability of the script. Things to remove:

• [x] function default values printed without spaces around = (need to have two code paths now)
• [x] enum types not having .name property (being different from enum.Enum, requiring a different code path)
• [x] checks for enum value docstrings being the same as enum type docstrings (2.3 can have docstrings for enum values as well, 2.2 makes both the same) no extra code for this (it's implicitly using empty doc for enum values), but there's a TODO for parsing 2.4 enum values

[mosra]

Done in 71fb96e7636377b12c33c404a931adb56668a370. The CI now tests 2.3, 2.4.3, 2.5.0 and 2.8.0.