Python: Copy Python extractor to codeql repo

python/extractor/blib2to3/README

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+This code is derived from the black code formatter, 
+which itself was derived from the lib2to3 package in the Python standard library.
+
+We (Semmle) have modified this further to ease conversion to our multi-version AST.
+
+Original README from black:
+
+A subset of lib2to3 taken from Python 3.7.0b2.
+Commit hash: 9c17e3a1987004b8bcfbe423953aad84493a7984
+
+Reasons for forking:
+- consistent handling of f-strings for users of Python < 3.6.2
+- backport of BPO-33064 that fixes parsing files with trailing commas after
+  *args and **kwargs
+- backport of GH-6143 that restores the ability to reformat legacy usage of
+  `async`
+- support all types of string literals
+- better ability to debug (better reprs)
+- INDENT and DEDENT don't hold whitespace and comment prefixes
+- ability to Cythonize