Fix python_full_version marker conversion to constraint

[sdispater]

I stumbled upon this one when trying to update poetry-core.

Basically, we would get the following error:

  SolverProblemError

  The current project's Python requirement (>=2.7,<2.8 || >=3.5,<4.0) is not compatible with some of the required packages Python requirement:
    - pre-commit requires Python >=3.6.1, so it will not be satisfied for Python >=2.7,<2.8 || >=3.5,<3.6.1

The root cause was that get_python_constraint_from_marker() was generating empty constraints for python_full_version markers since we were only checking python_version markers.

Resolves: python-poetry/poetry#2950