textproc/py-textfsm: Update to 2.1.0

• Switch to the PEP517 build framework.

• Add TEST_ENV to ensure that tests are run against the new copy of the module in the staging directory and remove superfluous TEST_DEPENDS as it's already set via "USE_PYTHON=pytest" while I'm here.

Changelogs since 1.1.0:

https://github.com/google/textfsm/releases/tag/v2.1.0
https://github.com/google/textfsm/releases/tag/v2.0.0