make a tzinfo class at the module level instead of inlining (fixes #81)

[beckjake]

Fixes #81 by creating a generic class with a name and an offset, and using instances of that for timezones.

I'm not clear on why snowflake uses type inline to create a fresh class for each timezone, but it breaks picking of results, which is very annoying and means we have to wrap all results with code that does basically this if we want to pickle anything.

This pattern comes straight from the python docs, check out the "FixedOffset" class defined on this page

[drewbanin]

Is anyone able to take a look at this one?

[smtakeda]

no worry, this is in the queue.

[smtakeda]

Writing a test case:

def test_pickle_timestamp_tz(tmpdir, conn_cnx):
    tmp_dir = str(tmpdir.mkdir('pickles'))
    output = os.path.join(tmp_dir, 'tz.pickle')
    expected_tz = None
    with conn_cnx() as con:
        for rec in con.cursor().execute("select '2019-08-11 01:02:03.123 -03:00'::TIMESTAMP_TZ"):
            expected_tz = rec[0]
            with open(output, 'wb') as f:
                pickle.dump(expected_tz, f)
                print(expected_tz)
                print(type(expected_tz))

    with open(output, 'rb') as f:
        read_tz = pickle.load(f)
        print(read_tz)
        assert expected_tz == read_tz

But got this error:

>           read_tz = pickle.load(f)
E           TypeError: __init__() missing 2 required positional arguments: 'name' and 'tzoffset_minutes'

[smtakeda]

This looks like working well.
https:/stub42/pytz/blob/master/src/pytz/__init__.py#L391

pytz.FixedOffset

[smtakeda]

@beckjake and @drewbanin can you review #191 ?

[drewbanin]

Hey @smtakeda - this looks really great! I didn't know about pytz.FixedOffset. I gave this patch a spin locally and it appears to work really well :)

[smtakeda]

working on other branch.