sage-bootstrap: Drop support for Python 2, < 3.5; port sagemath-environment to Python >= 3.5

[mkoeppe]

... where $x$ is to be determined based on the default Python on the oldest supported distributions

• see Meta-ticket: Remove support for outdated distros #32074

This will allow us to run the distribution sagemath-environment (providing sage.features) at configure time (in sage-bootstrap-python). Then we will be able to write spkg-configure scripts in Python instead of in autoconf.

But it looks like python2 will still be with us for a while:

$ docker run -it ubuntu:xenial  
root@64aeb392adbd:/# python
bash: python: command not found
root@64aeb392adbd:/# apt-get update && apt-get install python
The following additional packages will be installed:
  file libexpat1 libffi6 libmagic1 libpython-stdlib libpython2.7-minimal libpython2.7-stdlib libsqlite3-0 libssl1.0.0 mime-support python-minimal python2.7 python2.7-minimal
root@64aeb392adbd:/# apt-get install python3
The following additional packages will be installed:
  dh-python file libexpat1 libmagic1 libmpdec2 libpython3-stdlib libpython3.5-minimal libpython3.5-stdlib libsqlite3-0 libssl1.0.0 mime-support python3-minimal python3.5 python3.5-minimal

$ docker run -it ubuntu:bionic
root@174c6629e897:/# python
bash: python: command not found
root@174c6629e897:/# python3
bash: python3: command not found
root@174c6629e897:/# apt-get update && apt-get install python
The following additional packages will be installed:
  file libexpat1 libmagic-mgc libmagic1 libpython-stdlib libpython2.7-minimal libpython2.7-stdlib libreadline7 libsqlite3-0 libssl1.1 mime-support python-minimal python2.7 python2.7-minimal readline-common xz-utils
root@174c6629e897:/# apt-get install python3
The following additional packages will be installed:
  file libexpat1 libmagic-mgc libmagic1 libmpdec2 libpython3-stdlib libpython3.6-minimal libpython3.6-stdlib libreadline7 libsqlite3-0 libssl1.1 mime-support python3-minimal python3.6 python3.6-minimal readline-common xz-utils

$ docker run -it ubuntu:focal                                                                                                      
root@2e9da4950757:/# apt-get update && apt-get install python
Note, selecting 'python-is-python2' instead of 'python'
The following additional packages will be installed:
  file libexpat1 libmagic-mgc libmagic1 libpython2-stdlib libpython2.7-minimal libpython2.7-stdlib libreadline8 libsqlite3-0 libssl1.1 mime-support python2 python2-minimal python2.7 python2.7-minimal readline-common xz-utils
root@2e9da4950757:/# apt-get install python3
The following additional packages will be installed:
  file libexpat1 libmagic-mgc libmagic1 libmpdec2 libpython3-stdlib libpython3.8-minimal libpython3.8-stdlib libreadline8 libsqlite3-0 libssl1.1 mime-support python3-minimal python3.8 python3.8-minimal readline-common xz-utils