[BUG] Avoid using pytz for timezone parsing

[sdebruyn]

Is this a new bug in dbt-date?

• [x] I believe this is a new bug in dbt-date
• [x] I have searched the existing issues, and I could not find an existing issue for this bug

I am maintaining tsql_utils, the shims for popular dbt packages such as dbt_date for all T-SQL databases. The timezones in T-SQL are a bit peculiar and they don't support the ISO timezones. This means that the supported timezones are usually not able to be parsed with pytz. In the integration tests I currently have to override get_test_dates() just to remove the usage of pytz. It would be helpful if this was split up in a submacro or if there was no usage of pytz at all.

This is also the case for the datetime macro in modules_datetime which is a bigger problem since that's not just a test :)