Add variable to allow the tox version to be specified.

[samaloney]

So the idea here is you could set a parameter in the azure-pipline.yml config e.g. toxverspec: <0.4 but I'm not really sure what I'm doing here 🤔

[bsipocz]

Could you add a test for this in a new PR? That would help downstream to adopt the usage more than than the docs. Thanks!

[samaloney]

Yea I guess just add another stanza to the azure-pipelines.yml like and test it does what it's supposed to?

https://github.com/OpenAstronomy/azure-pipelines-templates/blob/deefa6efef819d63b10a09e41009d7fadd243ed5/azure-pipelines.yml#L38-L42

[bsipocz]

I ended up using the feature here, so a test along these lines should work here, too: https://github.com/astropy/pytest-filter-subpackage/pull/12/files

[samaloney]

I've added a test in a new PR #87