Complex number support

[chillenb]

Changes

Fixes #248

Adds support for complex numbers under tag 43000.

Checklist

If this is a user-facing code change, like a bugfix or a new feature, please ensure that
you've fulfilled the following conditions (where applicable):

• You've added tests (in tests/) added which would fail without your patch
• You've updated the documentation (in docs/, in case of behavior changes or new
  features)
• You've added a new changelog entry (in docs/versionhistory.rst).

If this is a trivial change, like a typo fix or a code reformatting, then you can ignore
these instructions.

Updating the changelog

If there are no entries after the last release, use **UNRELEASED** as the version.
If, say, your patch fixes issue #999, the entry should look like this:

- Fixed big bad boo-boo in the encoder (#999 <https://github.com/agronholm/cbor2/issues/999>_; PR by @yourgithubaccount)

If there's no issue linked, just link to your pull request instead by updating the
changelog after you've created the PR.

[coveralls]

coverage: 93.248% (-0.3%) from 93.542%
when pulling 1c4da62 on chillenb:complex
into d9cee77 on agronholm:master.

[chillenb]

Not sure what's happening with the fuzzer. It doesn't look like it ran properly?

[agronholm]

Not sure what's happening with the fuzzer. It doesn't look like it ran properly?

Don't worry about that. It never worked right, and I haven't been able to reach the person who added it. I haven't decided what to do about it yet.