Drop py34, add GitHub Actions and a tox env for publishing

[dmtucker]

Close #46

py34 has reached EOL, and tox-gh-actions does not support it.

Dropping support should not affect the vast majority of active users.
In the last 30 days, py34 accounted for only 0.003% of pytest-mypy downloads:

$ pypinfo pytest-mypy pyversion
Served from cache: False
Data processed: 117.47 GiB
Data billed: 117.47 GiB
Estimated cost: $0.58

| python_version | download_count |
| -------------- | -------------- |
| 3.7            |         56,011 |
| 3.8            |         18,070 |
| 3.6            |         15,508 |
| 3.9            |            678 |
| 3.5            |            324 |
| 3.10           |             90 |
| 2.7            |             28 |
| 3.4            |              3 |
| Total          |         90,712 |

Switching to GitHub Actions should more permanently resolve #46; however, we can't use it to replace Travis CI entirely without @dbader's help (because admin privileges are required to set repo Secrets, which we would need for PyPI creds e.g. https://github.com/dmtucker/backlog/blob/10701deae4fde379d281808184c3c715b53a980a/.github/workflows/publication.yml#L15-L16)

[dmtucker]

Hmm... Actions don't appear to be running 🤔 It works in my fork, though, maybe PR checks begin after the workflow has been committed for the first time?