CI not using set Python versions

[jacobtomlinson]

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