Inadvertently, testing is using the previous release

[Richard Elkins]

Blimpy testing needs setigen for regression testing.
Setigen needs blimpy to save data in Filterbank or HDF5 files.

So, when the blimpy dependency of setigen is installed and blimpy is not yet installed, testing proceeds with the previous release level from pypi.

Solution:
Take setigen out of requirements.txt and put it and all the other test requirements in requirements_test.txt (new file).

Impacted files:

• .github/workflows/python_tests.yml
• Dockerfile