Tighten bytes/str boundaries and remove unnecessary coercing

[jdufresne]

Now that the project is Python 3 only, the boundaries between bytes and
Unicode strings is more explicit.

This allows removing several unnecessary force_bytes() and
force_unicode() calls that handled differences between Python 2 and
Python 3. All uses of force_unicode() have been removed.

For values that are known to be bytes, use .decode() instead. For
values are that known to be str, use .encode() instead. This strategy
makes the type explicit and reduces a function call.

Key handling continues to use force_bytes() to allow callers to pass
either bytes or str.

To help enforce bytes/str handling in the future, the -b option is
passed to Python when testing. This option will emit a warning if bytes
and str are improperly mixed together.