Always close the zone.tab file
    
    Otherwise we get this in setuptools build with pytest 6.2+:
    
        >               warnings.warn(pytest.PytestUnraisableExceptionWarning(msg))
        E               pytest.PytestUnraisableExceptionWarning: Exception ignored in: <_io.FileIO [closed]>
        E
        E               Traceback (most recent call last):
        E                 File "/usr/lib/python3.9/site-packages/pytz/__init__.py", line 522, in <module>
        E                   common_timezones = [l.split()[2]
        E               ResourceWarning: unclosed file <_io.TextIOWrapper name='/usr/share/zoneinfo/zone.tab' mode='r' encoding='UTF-8'>
        /usr/lib/python3.9/site-packages/_pytest/unraisableexception.py:78: PytestUnraisableExceptionWarning