Provide access to average loss values from each epoch

[52b640]

The purpose of these changes is to provide access to the average loss values from each epoch of model-fitting.

Some of the included Cython changes are incidental: They were necessary for me to build and install lightfm for Python 3.10, the only version to which I have easy access.

Ostensibly related to: #275, #539, #601

[theMasix]

:(((