docs: Fix a few typos

[timgates42]

There are small typos in:

• docs/conf.py
• localflavor/za/forms.py
• tests/test_bg.py

Fixes:

• Should read that rather than shat.
• Should read simplistic rather than simlistic.
• Should read checksum rather than cheksum.

Semi-automated pull request generated by https://github.com/timgates42/meticulous/blob/master/docs/NOTE.md

[benkonrath]

Thanks!