Better support macOS, where /usr/share/zoneinfo/posix doesn't exist

[ccaviness]

In this case, iterate through the files in /usr/share/zoneinfo, skipping
unreadable files.

This allows datetime_tz.datetime_tz.smartparse to work in most cases.

Currently:

❯ python3
Python 3.6.8 (v3.6.8:3c6b436a57, Dec 24 2018, 02:04:31) 
[GCC 4.2.1 Compatible Apple LLVM 6.0 (clang-600.0.57)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import datetime_tz
>>> datetime_tz.datetime_tz.smartparse('Feb 22 00:30:04')
/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/datetime_tz/__init__.py:335: UserWarning: We detected no matches for your /etc/localtime.
  warnings.warn("We detected no matches for your /etc/localtime.")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/datetime_tz/__init__.py", line 606, in smartparse
    default = dt.replace(hour=0, minute=0, second=0, microsecond=0)
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/datetime_tz/__init__.py", line 564, in replace
    replaced, tzinfo=tzinfo or self.tzinfo.zone, is_dst=is_dst)
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/datetime_tz/__init__.py", line 425, in __new__
    tzinfo = _tzinfome(kw.pop("tzinfo"))
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/datetime_tz/__init__.py", line 100, in _tzinfome
    assert tzinfo.zone in pytz.all_timezones
AssertionError

With this commit:

❯ python3
Python 3.6.8 (v3.6.8:3c6b436a57, Dec 24 2018, 02:04:31) 
[GCC 4.2.1 Compatible Apple LLVM 6.0 (clang-600.0.57)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import datetime_tz
>>> datetime_tz.datetime_tz.smartparse('Feb 22 00:30:04')
/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/datetime_tz/__init__.py:326: UserWarning: Skipping posixrules because not in pytz database.
  warnings.warn("Skipping %s because not in pytz database." % tzname)
/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/datetime_tz/__init__.py:338: UserWarning: We detected multiple matches for your /etc/localtime. (Matches where [<DstTzInfo 'America/New_York' LMT-1 day, 19:04:00 STD>, <DstTzInfo 'US/Eastern' LMT-1 day, 19:04:00 STD>])
  "(Matches where %s)" % matches)
datetime_tz(2021, 2, 22, 0, 30, 4, tzinfo=<DstTzInfo 'America/New_York' EST-1 day, 19:00:00 STD>)
>>>