Use Conda envs in all CI jobs

[btjanaka]

Description

We encountered an issue in #366 where different Python instances caused swig to not be found. To prevent this and similar issues in the future, this PR creates Conda envs to ensure that we always / are more likely to use the correct version of Python.

TODO

Questions

• [x] Should we use Tox?
  • For now, I believe Tox would introduce too much complexity. All we want is to create a virtual env for each job. Tox may be able to do this with less boilerplate, but we would also incur overhead to learn how to use it properly. Conda envs also more closely match our audience of researchers.
  • Note that we did have Tox originally, but we removed it in #143 to simplify our workflows.

Status

• [x] I have read the guidelines in CONTRIBUTING.md
• [x] I have formatted my code using yapf
• [x] I have tested my code by running pytest
• [x] I have linted my code with pylint
• [x] I have added a one-line description of my change to the changelog in HISTORY.md
• [x] This PR is ready to go