Refactor PluggableTransport.getTransportLine() to remove `keyid=`.

 * FIXES #10559
 * FIXES a bunch of PEP8 problems.
 * CHANGE getTransportLine() to use `' '.join()` rather than formatting the
   bridge line inside the return statement. It's faster and safer.
Make the 'Bridge ' prefix optional in PluggableTransport.getTransportLine().
Merge branch 'fix/10559-keyid-in-pt-lines' into develop