Python support for dependabot-core.

We rely on pyenv to manage Python's versions.

Updating the list of known versions might be tricky, here are the steps:

1. Update the pyenv version in the Dockerfile, you may use a commit hash if a new pyenv version is not released yet.
2. Update the pyenv global version in the Dockerfile. We always use the latest (and greatest) Python version.
3. Update the list of known Python versions in language.rb (PRE_INSTALLED_PYTHON_VERSIONS_RAW).
4. Fix any broken tests.

Example PR that does all these things.

1. Start a development shell

$ bin/docker-dev-shell python

2. Run tests

   [dependabot-core-dev] ~ $ cd python && rspec