Added Language level configuration.

Changes:

Configured project with services to handle language specifications. Added Python2 print parsing.
Removed CollectIntelliJDependencies.java.
Added language level to PythonParser builder.
Added language level to assertions.
Added whitespace assertions to parser tests.
Revised parsePythonSource in IntelliJUtils to cover null PyFiles.
Added PythonVersion marker.
Updated FindPythonSources to include the PythonVersion of Py.CUs.

openrewrite / rewrite-python