cmake: switch python & boost-python search to py3

Python 2 is EOL now, and Python 3 has been available in distros for
years already, so it is time to switch exclusively to Python 3.

The actual changes to the BoostPython module are:
- look for the python3 boost component (instead of python): while boost
  upstream does not provide it, most of the distros do; the alternative
  would be to look for the pythonXY components, which is not ideal...
- look for the python libraries >= 3, setting as additional versions
  for the cmake module all the non-EOL Python 3 versions
- set as Python versions for pkg-config all the non-EOL ones, using the
  embed version starting from Python 3.8 [1]

[1] https://bugs.python.org/issue36721

BUG: 419237
FIXED-IN: 20.07.80