Add a pre-commit configuration

[sajith]

Following up the discussion on #202, adding a pre-commit configuration. The way to use this would be with:

$ pip install pre-commit
$ pre-commit install 

With this, a git commit will trigger the checks defined in .pre-commit-config.yml. This way, black and isort checks will be done on the local setup. Instead of CI nagging us about formatting, a pre-commit check will nag us!

Perhaps this can be (optionally) combined with the third party pre-commit.ci service, if we want the formatting changes to be auto-applied on repository side.

[coveralls]

Pull Request Test Coverage Report for Build 5649506150

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 20.728%

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Totals
Change from base Build 5647124708:	0.0%
Covered Lines:	927
Relevant Lines:	3998

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💛 - Coveralls