Provide ansible extra

[KamilaBorowska]

Fixes #947.

Thanks for creating a PR! Here's a quick checklist to pay attention to:

• Please add an entry to docs/changelog.rst as appropriate.

• Has some new parameter been added or semantics modified somehow? Please
  ensure relevant documentation is updated in docs/ansible.rst and
  docs/api.rst.

• If it's for new functionality, is there at least a basic test in either
  tests/ or tests/ansible/ covering it?

• If it's for a new connection method, please try to stub out the
  implementation as in tests/data/stubs/, so that construction can be tested
  without having a working configuration.

[KamilaBorowska]

Right, that won't work because for 2.10 the package is called ansible-base, not ansible-core.

[KamilaBorowska]

Eh, I guess I will call it ansible-core extra instead, and accept it won't match ansible-base. For my use-case of preventing ansible updates before mitogen gets updated it's acceptable.

[moreati]

/AzurePipelines run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[KamilaBorowska]

Updated the changelog to put this change in "Unreleased" section.

[moreati]

If you're till interested, there may be a way this could be work across ansible-base & ansible-core.

1. Release distributions of a new project (e.g. superluminiferous-aether) on PyPI with <major.minor> versions that correspond to ansible-base/ansible-core version and that require that corresponding ansible-base/ansible-core <major.minor>
2. Have mitogen[ansible] require superluminiferous-aether >= 2.10, <= 2.16.

I'm not sure yet, if the benefit outweighs the complexity.