Several tests failed under Python 3.7

I wanted to build RPM package but I found some tests are failed on 3.7 (Fedora)

```

check_astunparse (unittest.loader._FailedTest) ... ERROR

test_fast_compare (tests.test_misc.FastCompareTestCase) ... ok

test_get_mat_mult (tests.test_misc.GetSymbolTestCase) ... ok

test_aliases (tests.test_misc.PublicAPITestCase) ... ok

test_codegen_as_submodule (tests.test_misc.PublicAPITestCase) ... FAIL

test_codegen_from_root (tests.test_misc.PublicAPITestCase) ... ok

test_convert_stdlib (tests.test_rtrip.RtripTestCase) ... WARNING:root:    calculating dump -- bad

WARNING:root:    calculating dump -- bad

WARNING:root:    calculating dump -- bad

WARNING:root:    calculating dump -- bad

WARNING:root:    calculating dump -- bad

WARNING:root:

Files failed to round-trip to AST:

WARNING:root:    /usr/lib64/python3.7/dataclasses.py

WARNING:root:    /usr/lib64/python3.7/netrc.py

WARNING:root:    /usr/lib64/python3.7/warnings.py

WARNING:root:    /usr/lib64/python3.7/asyncio/base_events.py

WARNING:root:    /usr/lib64/python3.7/test/support/testresult.py

FAIL

check_expressions (unittest.loader._FailedTest) ... ERROR



======================================================================

ERROR: check_astunparse (unittest.loader._FailedTest)

----------------------------------------------------------------------

ImportError: Failed to import test module: check_astunparse

Traceback (most recent call last):

  File "/usr/lib64/python3.7/unittest/loader.py", line 154, in loadTestsFromName

    module = __import__(module_name)

  File "/builddir/build/BUILD/astor-0.7.1/tests/check_astunparse.py", line 8, in <module>

    import test_code_gen

ModuleNotFoundError: No module named 'test_code_gen'





======================================================================

ERROR: check_expressions (unittest.loader._FailedTest)

----------------------------------------------------------------------

ImportError: Failed to import test module: check_expressions

Traceback (most recent call last):

  File "/builddir/build/BUILD/astor-0.7.1/tests/check_expressions.py", line 44, in <module>

    mymod = importlib.import_module(mymodname)

  File "/usr/lib64/python3.7/importlib/__init__.py", line 127, in import_module

    return _bootstrap._gcd_import(name[level:], package, level)

  File "<frozen importlib._bootstrap>", line 1006, in _gcd_import

  File "<frozen importlib._bootstrap>", line 983, in _find_and_load

  File "<frozen importlib._bootstrap>", line 965, in _find_and_load_unlocked

ModuleNotFoundError: No module named 'all_expr_3_7'



During handling of the above exception, another exception occurred:



Traceback (most recent call last):

  File "/usr/lib64/python3.7/unittest/loader.py", line 154, in loadTestsFromName

    module = __import__(module_name)

  File "/builddir/build/BUILD/astor-0.7.1/tests/check_expressions.py", line 47, in <module>

    import build_expressions

ModuleNotFoundError: No module named 'build_expressions'





======================================================================

FAIL: test_codegen_as_submodule (tests.test_misc.PublicAPITestCase)

----------------------------------------------------------------------

Traceback (most recent call last):

  File "/builddir/build/BUILD/astor-0.7.1/tests/test_misc.py", line 40, in test_codegen_as_submodule

    import astor.codegen

AssertionError: DeprecationWarning not triggered



======================================================================

FAIL: test_convert_stdlib (tests.test_rtrip.RtripTestCase)

----------------------------------------------------------------------

Traceback (most recent call last):

  File "/builddir/build/BUILD/astor-0.7.1/tests/test_rtrip.py", line 24, in test_convert_stdlib

    self.assertEqual(result, [])

AssertionError: Lists differ: ['/usr/lib64/python3.7/dataclasses.py', '/[159 chars].py'] != []



First list contains 5 additional elements.

First extra element 0:

'/usr/lib64/python3.7/dataclasses.py'



+ []

- ['/usr/lib64/python3.7/dataclasses.py',

-  '/usr/lib64/python3.7/netrc.py',

-  '/usr/lib64/python3.7/warnings.py',

-  '/usr/lib64/python3.7/asyncio/base_events.py',

-  '/usr/lib64/python3.7/test/support/testresult.py']



----------------------------------------------------------------------

Ran 44 tests in 14.885s



FAILED (failures=2, errors=2)

Test failed: <unittest.runner.TextTestResult run=44 errors=2 failures=2>

Expression list does not exist -- building

error: Test failed: <unittest.runner.TextTestResult run=44 errors=2 failures=2>

error: Bad exit status from /var/tmp/rpm-tmp.ZeKR6g (%check)

```



See more on builder-live.log in https://copr-be.cloud.fedoraproject.org/results/rpisarev/tensorflow/fedora-30-x86_64/00869299-python-astor/

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