This commit refactors the codebase to consistently use Python 3.6
f-strings for string concatenation and interpolation.

BREAKING CHANGE: This update increases the minimum required Python
version from 3.5 to 3.6. Python 3.5 reached end-of-life in September
2020 (~4 years ago) and has been dropped from most distributions, so the
impact on users should be minimal.

They were for an entirely different project.

Signed-off-by: Adam Williamson <awilliam@redhat.com>

Signed-off-by: Adam Williamson <awilliam@redhat.com>

Signed-off-by: Adam Williamson <awilliam@redhat.com>

- Center title to match the logo
- Change logo image to the PNG version, as SVG seems to be unsupported by PyPI:
  https://pypi.org/project/mwclient/
- Remove Landscape.io badge, as the service doesn't seem to be working:
  - https://landscape.io/github/mwclient/mwclient
  - https://github.com/landscapeio/landscape-issues/issues
- Add alt attribute to logo image

simplejson is now a requirement installed by easy_install, pip
or other toos. ReadMe was converted from MarkDown to reStructuredText
to make it usable at PyPI while still being supported by GitHub.