Ignore Cython debug symbols (#3330)

Cython extension modules built with `gdb_debug=True` spit out debug symbols in the `cython_debug` directory at the top level of the project. The files in this directory contain hardcoded paths and are not shareable/meaningful across environments, so I think it makes sense to include them in a default Python .gitignore.

Python.gitignore

@@ -133,3 +133,6 @@ dmypy.json
 
 # pytype static type analyzer
 .pytype/
+
+# Cython debug symbols
+cython_debug/