Indicate that pytz is the accepted option for tz

Squashed commit of the following:

commit e4a7e7c
Author: Zander Hill <zander@xargs.io>
Date:   Tue Sep 25 13:13:46 2018 -0700

    Match to 74 chars line length

commit 547c878
Author: Zander Hill <zander@xargs.io>
Date:   Tue Sep 25 13:05:26 2018 -0700

    Add backticks to indicate pytz is a library

commit d639cf4
Author: Zander Hill <zander@xargs.io>
Date:   Tue Sep 25 12:19:41 2018 -0700

    Indicate that pytz is the accepted option for tz

    @timvisher Here's the documentation patch for using pytz as the canonical representation of timezones in Singer taps.

docs/BEST_PRACTICES.md

@@ -36,6 +36,9 @@ Good:
 Bad:
  - 2017-01-01 00:00:00
 
+When parsing datetimes or converting between timezones, the library
+accepted for use by singer taps is `pytz`. This project has more accurate
+timezone data than the builtin datetime.timezone.
 
 ## Logging and Exception Handling