Adds support for BG Electrical EHC31 Extension Lead (0x6480)

Context

This PR adds support for the BG Electrical EHC31, a smart extension lead (or power-strip) with device type 0x6480.

I'm not a Python dev, but this small change seems to work! I tried to stick to existing conventions where I noticed them.

I've tested this extensively locally with my own little test script, and noted down in the comment for this new device's set_state() method my findings just in case they'd be of use to anyone else.

Note: Since I branched off master, this PR can't be automatically merged. If it'd be easier, I can re-create my fork from the new_product_ids branch and re-open this PR.

Proposed change

Added new device class of ehc31, which extends the existing bg1 class to devices.py
Added ehc31 definition to SUPPORTED_TYPES with device type 0x6480
Bit of code-formatting in the SUPPORTED_TYPES object (thanks VSCode/Black)
Added EHC31 to the supported devices list in README.

Type of change

[ ] Dependency upgrade
[ ] Bugfix (non-breaking change which fixes an issue)
[x] New device
[ ] New product id (the device is already supported with a different id)
[ ] New feature (which adds functionality to an existing device)
[ ] Breaking change (fix/feature causing existing functionality to break)
[ ] Code quality improvements to existing code or addition of tests
[ ] Documentation

Additional information

This PR fixes issue: fixes #
This PR is related to:
Link to documentation pull request:

Checklist

[x] The code change is tested and works locally.
[x] The code has been formatted using Black.
[ ] The code follows the Zen of Python. Probably!?
[x] I am creating the Pull Request against the correct branch.
[x] Documentation added/updated.

mjg59 / python-broadlink