add dmypy.json - without initial '.', to Python.gitignore.

Older versions of dmypy used 'dmypy.json' without the initial '.'.
This should be added to Python.gitignore for those using older versions
of MyPy.

Python.gitignore

@@ -103,3 +103,5 @@ venv.bak/
 # mypy
 .mypy_cache/
 .dmypy.json
+dmypy.json
+