davos.implementations.ipython_common._check_conda_avail_helper broken for IPython<7.3

[paxtonfitzpatrick]

The _check_conda_avail_helper() function checks whether the conda executable is available and accessible from the Python interpreter by running conda list IPython. If successful, it returns the captured stdout from the command so the path to the active conda environment can be parsed from it.

The ipython_common implementation of this helper function runs the shell command via the %conda line magic (<ipython_shell>.run_line_magic()) as a workaround for #10. However, the %conda line magic wasn't implemented until IPython v7.3 (changelog), so the command will fail for older versions, which davos incorrectly interprets as the conda executable not being available.

I think the best (only?) way to deal with this is to simply reimplement IPython's approach to determining the kernel's conda environment, here.

Note: for now, the test for _check_conda_avail_helper in test_ipython_common.ipynb is restricted to IPython>=7.3 until this is resolved.

[paxtonfitzpatrick]

test_conda_env_fset_raises_noenv in test_implementations.ipynb is also restricted to IPython>=7.3 until this is resolved.