Fix name of unittest @classmethod method

[mdutoo]

see https://docs.python.org/3/library/unittest.html#class-and-module-fixtures

else it is never called, and both tests fail using Python 3.5 :

(.venv) $ python -m unittest -v tests/test_mocked.py
test_simple_csv (tests.test_mocked.TestImport) ... /home/mdutoo/dev/ozwillo/workspace/datapusher/tests/test_mocked.py:27: ResourceWarning: unclosed file <_io.TextIOWrapper name='/home/mdutoo/dev/ozwillo/workspace/datapusher/tests/static/simple.csv' mode='r' encoding='UTF-8'>
return open(join_static_path(filename)).read()
ResourceWarning: Enable tracemalloc to get the object allocation traceback
ERROR
test_wrong_api_key (tests.test_mocked.TestImport) ... ERROR

======================================================================
ERROR: test_simple_csv (tests.test_mocked.TestImport)

Traceback (most recent call last):
File "/home/mdutoo/dev/ozwillo/workspace/datapusher/.venv/lib/python3.8/site-packages/httpretty/core.py", line 1627, in wrapper
return test(*args, **kw)
File "/home/mdutoo/dev/ozwillo/workspace/datapusher/tests/test_mocked.py", line 81, in test_simple_csv
'api_key': self.api_key,
AttributeError: 'TestImport' object has no attribute 'api_key'

======================================================================
ERROR: test_wrong_api_key (tests.test_mocked.TestImport)

Traceback (most recent call last):
File "/home/mdutoo/dev/ozwillo/workspace/datapusher/.venv/lib/python3.8/site-packages/httpretty/core.py", line 1627, in wrapper
return test(*args, **kw)
File "/home/mdutoo/dev/ozwillo/workspace/datapusher/tests/test_mocked.py", line 106, in test_wrong_api_key
'api_key': self.api_key,
AttributeError: 'TestImport' object has no attribute 'api_key'

---

Ran 2 tests in 0.002s

FAILED (errors=2)