Bump pytz from 2018.5 to 2019.3

[dependabot-preview[bot]]

Bumps pytz from 2018.5 to 2019.3.

Commits

- [`3db1213`](https://github.com/stub42/pytz/commit/3db12139c6da6ae3fb2f6fb4988adc9db14e3838) Bump version numbers to 2019.3/2019c - [`6870ec1`](https://github.com/stub42/pytz/commit/6870ec1a032a8200cf2557a298b3132c7e1bfc9f) IANA 2019c - [`5e47a3e`](https://github.com/stub42/pytz/commit/5e47a3ec05c6dbc232fc867cec78ec4568f27c99) Squashed 'tz/' changes from 23a01523a..c5b665d49 - [`62f8720`](https://github.com/stub42/pytz/commit/62f872054dde69e5c510094093cd6e221d96d5db) Bump version numbers to 2019.2/2019b - [`a2c7caa`](https://github.com/stub42/pytz/commit/a2c7caaad0746a0bd787269a0ad5967281d84204) Adjust doctests to work with Python 3.8 - [`f417f53`](https://github.com/stub42/pytz/commit/f417f5398e4df1ef79c6c3835368f72b39a08af7) Use explicit pickle protocol versions in tests - [`0537ddc`](https://github.com/stub42/pytz/commit/0537ddc6a7724f10b59c78c9ca8c7ac7b9bc11af) Defer generating case-insensitive lookups - [`b9cb919`](https://github.com/stub42/pytz/commit/b9cb91934af36af63d6ca29a6cc0215c1489a6e5) Squashed 'tz/' changes from 14c73382c..09488fe23 - [`95c23a1`](https://github.com/stub42/pytz/commit/95c23a1f882a3d474362a906a388c5e85f59f23e) IANA 2019b - [`5b379f5`](https://github.com/stub42/pytz/commit/5b379f56c610b388c678d532dd303cb6f4b80aeb) Update dev README - Additional commits viewable in [compare view](https://github.com/stub42/pytz/compare/release_2018.5...release_2019.3)

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[dependabot-preview[bot]]

Superseded by #199.