Set cache key based on actual Python version

[pronovic]

Currently, the dependency cache key uses the matrix.python value, which is 3.8, 3.9, 3.10, etc. However, when Poetry sets up a the virtualenv, it's for a particular Python version (say 3.10.6). If the Python version changes (say, to 3.10.7), the cache is no longer valid, because the Python executable can no longer be found. The fix is to set the cache key based on the actual Python version that was used to create the virtualenv, and not the matrix Python version.

[coveralls]

Coverage remained the same at 98.117% when pulling 5f79750 on ken/cache-fixes into 66be4d5 on master.