README: new logo

New logo with 50% size and white background.

Fixes the error:

    Checking dist/bleak-0.22.3-py3-none-any.whl: FAILED
    ERROR    `long_description` has syntax errors in markup and would not be rendered on PyPI.
            line 5: Warning: Cannot scale image!
            Could not get size from "https://raw.githubusercontent.com/hbldh/bleak/master/Bleak_logo.png":
            Reading external files disabled.

Also, black text with dark mode GitHub wasn't very readable.

fix issue number

Merge pull request #1257 from hbldh/release/0.20.0

Release/v0.20.0