Replace nose (with pytest?), as it's in maintenance mode for a few years

[rominf]

Issue Summary

I'm a Fedora packager for python3-twilio. I've encountered an error during the build:

ERROR: InvocationError for command could not find executable nosetests

This comes from the fact that nose is apparently required for running the tests, but is not specified in tests/requirements.txt.

The obvious solution would be simply add it there, however, it is bad to add deprecated library (see: https://nose.readthedocs.io/en/latest, https://fedoraproject.org/wiki/Changes/DeprecateNose).

My proposal is to replace this invocation with pytest. I've made a patch in Fedora, and it works fine.

[childish-sambino]

Relates to #588

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