Change py.test to pytest

[Dovacody]

Description of proposed changes
After Careful review of the CONTRIBUTING.MD file, I found a typo on concerning pytest. Easy fix

Fixes #336

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.

[welcome]

💖 Thanks for opening this pull request! 💖

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[weiji14]

Good work @Dovacody! I didn't expect anyone to jump on so fast, it seems that there's a duplicate PR at #338 already. If you can find the other py.test by searching the project and fix that one too that'll be great!

[Dovacody]

Consider it done !

[weiji14]

Magic! Now 2 more things left to do:

1. You'll need to edit your initial post, and change the 'Fixes #' to 'Fixes [Easy typo fix] #dropthedot from py.test #336' so that the issue will be automatically closed when we merge in that pull request.

2. Update your branch by clicking on the "Update branch" button Oops, you beat me to it!

Oh, and you might want to change the title to Change py.test to pytest since it's not just the CONTRIBUTING.md file you're changing anymore.

[Dovacody]

Done and Done!

[welcome]

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[weiji14]

Awesome work @Dovacody! Just a small tip for your next Pull Request - make your changes on a 'patch' branch instead of working on the 'master' branch, as that could be problematic for fast moving projects that merge into 'master' often. Other than that, I hope you learnt something helpful today 😄