Add doctest to CI test job

[kadykov]

Fixes #798

[martindurant]

micromamba can be changed back to miniconda now

[martindurant]

601     >>> tf = intake.readers.catalogs.TensorFlowDatasetsCatalog().read()
UNEXPECTED EXCEPTION: TypeError("unsupported operand type(s) for |: 'NoneType' and '_UnionGenericAlias'")

suggests that the library (or its etils.epy dependency) isn't actually py38/39 compatible

[kadykov]

Probably there is something wrong with micromamba, because for Python 3.8 it chooses version 1.5.1 that requires-python = ">=3.9", and for Python 3.9 it installs version 1.6.0 that requires-python = ">=3.10".

[martindurant]

Probably there is something wrong with micromamba

Totally possible, I don't think it's that well supported now that conda uses the mamba solver internally.