Insert proper patch for py37+ support on dataclasses

The patch is proposed on https://github.com/ericvsmith/dataclasses/pull/161 on goes beyond what I did for this package in the past.

It (gracefully) falls-back to the built-in dataclasses on python 3.7+ instead of replacing that library.

This package was added here to stop pytorch=1.7 from complaining during installation. pytorch depends (wrongly) on dataclasses for any version of Python, whereas this should be done, only for python<3.7. A fix will only be available on pytorch=1.8 (but that may take another couple of weeks to show).

Closes bob/bob.ip.binseg#5