Fix documentation on testing

[ehrenfeu]

Documentation states that testing is done via nosetests:

https://github.com/CellProfiler/python-bioformats/blob/cac3116aa1abae7f9d973464a8bb937df362e7f6/docs/index.rst#L28-L39

This gave me a really hard time until I realized that tests were failing due to the fixture tmpdir not being provided by nose but only by pytest - which made me discover that commit 9fdf3401b359cf7ddd1c24c7c5efcabac3568c41 says "Use pytest" in its message and just the documentation is outdated.