Remove Pipfile from the repo

[sanketsaurav]

Currently Pipfile is used to freeze the dependencies used in the test workflow. Since the repo needs to be tested on multiple Python versions using tox, Pipfile isn't the ideal way to doing this because it works with only one Python version at a time.

The test dependencies can be managed better using the tests_require parameter in setup.py, which can then directly work with tox.

[immohsin]

claimed this issue

[sanketsaurav]

Closed in #2. Thanks @mohmum!