Wrong information in CONTRIBUTING.md

[harshitsinghai77]

Hey,

https://github.com/deta/deta-python/blob/master/CONTRIBUTING.md

It says

Run the tests

python tests.py

I don't think there's a tests.py file in the root directory.

[lemonyte]

Fixed by #69. The command is pytest tests (tests is the directory)