Add pre-commit config

[Mattwmaster58]

This PR adds a .pre-commit-config.yaml to be used by pre-commit to enforce rules and format files. Hooks:

• trailing-whitespace
  • removes trailing whitespace
• end-of-file-fixer
  • fixes files to end with newline
• check-yaml
  • checks that all YAML files are valid
• check-toml
  • checks that all TOML files are valid
• check-builtin-literals
  • checks that builtin literals (eg checks that {'i': 1} is used over dict(i=1))
• debug-statements
  • check for breakpoint() and other debugging imports
• isort
  • sorts imports nicely
• black
  • format with black

Also included:

• tox.ini change so that we simply check if the pre-commit conditions are met.
• CONTRIBUTING.md update so that people are encouraged to use pre-commit hooks

Enabling these hooks is as easy as installing pre-commit and running pre-commit install

[Mattwmaster58]

gah! accidentally committed some files that were formatted using black with an incorrect configuration.

Should be good now.

[Granitosaurus]

Looks good!