CI not using set Python versions

As noted in #138 it appears the CI is configured to test against Python 3.6 and 3.7. However things are getting upgraded to 3.8 every time.

This caused a problem as pytest was being pinned to 3.7 and 3.8 became the default Python version. But the current config would never have worked for 3.6 anyway as if it were correctly testing against 3.6 the pytest at 3.7 would have failed.

Things to fix here:
- Conda is complaining about the shell not being activated correctly. Despite `.bashrc` being sourced.
- The `conda create` in `bash_install.sh` isn't pinning the Python version correctly.
- We should also add 3.8 tests once above is fixed