Document how marbles compares to pytest

[thejunglejane]

Is your feature request related to a problem? Please describe.
Many people use pytest instead of unittest for testing their Python code, meaning that many people may ask “how does marbles compare to pytest?”

Describe the solution you'd like
A section in the docs comparing and contrasting marbles and pytest that helps readers make informed decisions about which tool to use. Many of the differences between marbles and pytest are differences between unittest and pytest. It is worthwhile to document our reasons for choosing to expand on unittest instead of pytest.