Line 21 of lib2to3/tests/pytree_idempotency.py:

import pgen2

This seems to be broken, because it should be:

import lib2to3.pgen2

(This makes the auto-requirements generator in ALT Sisyphus to generate a broken/unsatisfiable dependency on pgen2 when packaging this into an RPM.)

All other uses of pgen2 in Python 3.5.1 sources use other forms of import: either relative or the absolute one as I suggest:

$ git grep -F pgen2
lib2to3/btm_matcher.py:        # from .pgen2 import token // token.__dict__.items():
lib2to3/btm_utils.py:from .pgen2 import grammar, token
lib2to3/fixer_util.py:from .pgen2 import token
lib2to3/fixes/fix_apply.py:from ..pgen2 import token
lib2to3/fixes/fix_dict.py:from ..pgen2 import token
lib2to3/fixes/fix_except.py:from ..pgen2 import token
lib2to3/fixes/fix_filter.py:from ..pgen2 import token
lib2to3/fixes/fix_has_key.py:from ..pgen2 import token
lib2to3/fixes/fix_map.py:from ..pgen2 import token
lib2to3/fixes/fix_ne.py:from ..pgen2 import token
lib2to3/fixes/fix_next.py:from ..pgen2 import token
lib2to3/fixes/fix_numliterals.py:from ..pgen2 import token
lib2to3/fixes/fix_print.py:from ..pgen2 import token
lib2to3/fixes/fix_raise.py:from ..pgen2 import token
lib2to3/fixes/fix_throw.py:from ..pgen2 import token
lib2to3/fixes/fix_tuple_params.py:from ..pgen2 import token
lib2to3/fixes/fix_types.py:from ..pgen2 import token
lib2to3/fixes/fix_unicode.py:from ..pgen2 import token
lib2to3/fixes/fix_ws_comma.py:from ..pgen2 import token
lib2to3/patcomp.py:from .pgen2 import driver, literals, token, tokenize, parse, grammar
lib2to3/pgen2/__init__.py:"""The pgen2 package."""
lib2to3/pgen2/conv.py:from pgen2 import grammar, token
lib2to3/pgen2/token.py:#   originally monkeypatched in by pgen2.tokenize
lib2to3/pgen2/tokenize.py:from lib2to3.pgen2.token import *
lib2to3/pygram.py:from .pgen2 import token
lib2to3/pygram.py:from .pgen2 import driver
lib2to3/pytree.py:        # from .pgen2 import token // token.__dict__.items():
lib2to3/refactor.py:from .pgen2 import driver, tokenize, token
lib2to3/tests/pytree_idempotency.py:import pgen2
lib2to3/tests/pytree_idempotency.py:from pgen2 import driver
lib2to3/tests/pytree_idempotency.py:            except pgen2.parse.ParseError as err:
lib2to3/tests/support.py:from lib2to3.pgen2 import driver
lib2to3/tests/test_parser.py:from lib2to3.pgen2 import tokenize
lib2to3/tests/test_parser.py:from ..pgen2.parse import ParseError
lib2to3/tests/test_refactor.py:from lib2to3.pgen2 import token
lib2to3/tests/test_util.py:from lib2to3.pgen2 import token