Use a combination of travis and twine to publish to PyPI. New
publications will be made:

* When `master` is updated. This will trigger the publication of a
  the Python artifact versioned an iso-compliant build datetime. In this
  way, the cutting edge version will always be available via PyPI.
* When a new git tag is pushed. Tag pushes trigger the publication of
  a python artifact with the same version as the git tag, with the
  leading `v` stripped if present (`v1.2.3` becomes `1.2.3`).

Publications rely on Travis having a PYPI_PASSWORD environment set in
the project settings. See the Travis CI documentation for information on
[setting environment variables which containing sensitive data][1]. Make
extra sure the "Display value in build log" switch is OFF.

In addition to setting the previously mentioned `PYPI_PASSWORD`
environment variable, the owner of the PyPI `flatbuffers` repository
should, after merging this commit into master, add his own commit to
change `mikeholler` in `.travis/deploy-python.sh` to his username. It's
also recommended that the owner of `flatbuffers` use a separate account
in the unlikely event that the environment variable somehow becomes
compromised. Again, this is very unlikely, since the environment
variable is only set for "safe" builds approved by maintainers (not on
random pull requests).

[1]: https://docs.travis-ci.com/user/environment-variables/#Defining-Variables-in-Repository-Settings