Add a GitHub Action to make sure that an environment can be created from `requirements.txt`

[namurphy]

Since a requirements.txt file is being added in #5, we could add a GitHub Action to make sure that the environment can be resolved.

We should use the same dependency resolver that will be used to create the environment to be used in the summer school, which I presume will be pip (but might be conda, mamba, or uv).