Automate release upload

[dopplershift]

We should make our lives easier and automatically upload the sdist to PyPI when a new release is tagged. Stealing from our MetPy Travis config:

deploy:
  - provider: pypi
    user: dopplershift
    password:
      secure: <ENCRYPTED_PW>
    distributions: sdist
    upload_docs: no
    on:
      repo: SciTools/cartopy
      python: 3.6
      condition: [MAKE SURE WE ONLY DO IT ONCE]
      tags: true

We might also want to automatically update the docs, but I’m not clear on what the current process for that is.

[QuLogic]

Docs are put here: https://github.com/SciTools/scitools.org.uk I'm not sure if much else happens other than replacing the relevant directory.

[ajdawson]

I'm not sure if much else happens other than replacing the relevant directory.

Probably some fiddling with the version switcher to update where latest points to.

[ajdawson]

Regardless, we should definitely do an automated PyPI upload to save relying on an individual to do it.

[dopplershift]

sdist uploading (in theory) was done in #1163. I'll leave this open to remind about uploading docs.

[greglucas]

Want to try implementing this for the 0.18 release?

[greglucas]

Thanks, @dopplershift! This worked like a charm for this release.