only run publish workflow when creating new version tags

[wrboyce]

[coveralls]

Coverage remained the same at 73.529% when pulling 2795a474d48855e4f9ede70f76783e9957cf9ef2 on wrboyce:ci-releases into a1e842cf95e5bd99a84e89d5eaaed42ff82f57eb on zappa:master.

[javulticat]

Will this create only a py3 wheel in pypi or will it create a py2.py3 wheel (or both)? It looks like for 0.54.1, a py2.py3 wheel was also created, which seems incorrect, since we do not support Python 2. AFAIK, we should try to ensure that we are only publishing a py3 wheel. Historically, that's all we've published manually.

Examples:

0.54.1 (automated - py3 and py2.py3)

0.54.0 (manual - just py3)

[monkut]

Handled in https://github.com/zappa/Zappa/issues/1152

cloisng.