Bump pytz from 2019.3 to 2020.1

Bumps pytz from 2019.3 to 2020.1.

Commits

• 164d6af Fix reST lint
• 416edf6 Test against Python 3.8 and Python 3.9
• b25606a Bump version numbers to 2020.1/2020a
• 238b4f6 Base class for all errors
• fb8b146 Add flake8 settings
• a11229c IANA 2020a
• bc11842 Squashed 'tz/' changes from 765984a93..13af01673
• 7b1a844 Fix remaining references to README.txt
• f6fd618 Update README.md
• fd39cf1 Use .rst extension for reStructuredText
• Additional commits viewable in compare view