Remove support for legacy Python 2.7

[climbfuji]

Description

The top-level CMakeLists.txt is still looking for Python 2.7 if Python 3 cannot be found.

Solution

Make sure that every system that we are running on (tier-1 and tier-2) has Python 3 (they better do) - update the modulefiles if necessary. Remove check for Python 2.7 from top-level CMakeLists.txt, only look for Python 3.

Alternatives

Keep supporting Python 2, even though it is no longer developed and support will end soon.

Related to

See #357 (comment)