Adapt default values in docstrings to supported format

[kbattocchi]

Sphinx 3.2 checks default argument specifications more stringently (see e.g. https://github.com/sphinx-doc/sphinx/issues/8088), so the build was failing with that version of Sphinx. These changes should address the issue, except that our classes that derive from classes in external libraries can still have problems with inherited method docstrings (e.g. StatsModelsLinearRegression), so we either need to wait for sklearn to update their own classes or for Sphinx to relax the new restrictions.