use major/minor version of Python command being used if req_py_majver/req_py_minver are not specified

[boegel]

Important note: the part of the code being changed here is only relevant if Python is not included as a dependency, so it only affects a very limited amount of easyconfigs for Python software (mostly EasyBuild itself, but also PyYAML easyconfigs using the system toolchain, ReFrame, etc.).

The changes here fix the issue reported in easybuilders/easybuild-easyconfigs#9151, where installing an EasyBuild easyconfig (for example via eb --install-latest-eb-release) fails in the sanity check, because the installation is performed using the system python command, while the sanity check paths are derived using the Python version that is used to run EasyBuild with (basically because SYS_PYTHON_VERSION is used in allow_system_deps in the EasyBuild easyconfigs).

By specifying that the major/minor Python version of the Python being used by EasyBuild itself (via sys.version_info) as requirement when picking the python command, the correct python command is picked, and things work out correctly...

We're changing the default value for req_py_majver and req_py_minver here, but that's OK to do imho since i) this doesn't affect easyconfigs which already set a different major/minor version (see ReFrame easyconfigs for example), and ii) the current default of requiring Python 2.6 was sort of dodgy anyway (it worked well while EasyBuild only supported Python 2, but not anymore).

This not only fixes the issue for EasyBuild reported in easybuilders/easybuild-easyconfigs#9151, but also for PyYAML-3.13.eb for example (when the system python is Python 2.7, but EasyBuild itself is being run with Python 3.x).

[ocaisa]

LGTM