bug#39871: python-hacking build fails

[Maxim Cournoyer]

Build log extract:

--8<---------------cut here---------------start------------->8---
OS_STDERR_CAPTURE=${OS_STDERR_CAPTURE:-1} \
OS_TEST_TIMEOUT=${OS_TEST_TIMEOUT:-60} \
${PYTHON:-python} -m subunit.run discover -t ./ .  --load-list 
/tmp/guix-build-python-hacking-1.0.0.drv-0/tmp0b3_dkul
running=OS_STDOUT_CAPTURE=${OS_STDOUT_CAPTURE:-1} \
OS_STDERR_CAPTURE=${OS_STDERR_CAPTURE:-1} \
OS_TEST_TIMEOUT=${OS_TEST_TIMEOUT:-60} \
${PYTHON:-python} -m subunit.run discover -t ./ .  --load-list 
/tmp/guix-build-python-hacking-1.0.0.drv-0/tmput8xgx54
running=OS_STDOUT_CAPTURE=${OS_STDOUT_CAPTURE:-1} \
OS_STDERR_CAPTURE=${OS_STDERR_CAPTURE:-1} \
OS_TEST_TIMEOUT=${OS_TEST_TIMEOUT:-60} \
${PYTHON:-python} -m subunit.run discover -t ./ .  --load-list 
/tmp/guix-build-python-hacking-1.0.0.drv-0/tmpv10yr5p9
======================================================================
FAIL: hacking.tests.test_local.HackingTestCase.test_local_check
tags: worker-16
----------------------------------------------------------------------
Traceback (most recent call last):
  File 
"/tmp/guix-build-python-hacking-1.0.0.drv-0/hacking-1.0.0/hacking/tests/test_local.py",
 line 35, in test_local_check
    checker.check_all()
  File 
"/gnu/store/nk07kycilsspl02pg8jp3j3n9l6c6x6a-python-pep8-1.5.7/lib/python3.7/site-packages/pep8.py",
 line 1438, in check_all
    self.check_logical()
  File 
"/gnu/store/nk07kycilsspl02pg8jp3j3n9l6c6x6a-python-pep8-1.5.7/lib/python3.7/site-packages/pep8.py",
 line 1328, in check_logical
    (start_row, start_col) = mapping[0][1]
TypeError: 'NoneType' object is not subscriptable
Ran 185 tests in 0.100s
FAILED (id=0, failures=1)
command "python" "-c" "import setuptools, 
tokenize;__file__='setup.py';f=getattr(tokenize, 'open', 
open)(__file__);code=f.read().replace('\\r\\n', 
'\\n');f.close();exec(compile(code, __file__, 'exec'))" "test" failed with 
status 1
builder for 
`/gnu/store/cb7v3aq2a26350ha85k5lynmgzbsyaip-python-hacking-1.0.0.drv' failed 
with exit code 1
--8<---------------cut here---------------end--------------->8---

On master c1febbbf94ee794d7a97dbde12102634f2b13529.