Now that Python 2 is not supported, the bytes/str boundaries are more
clear and explicit. Using six.ensure_* methods for backwards
compatibility is no longer necessary as the types are known and verified
using mypy.

One exception is tests/lib/wheel.py which allows tests to pass test
setup data as either bytes or str.

The module operations.install.wheel also remains untouched as it is
especially delicate to bytes/str mixups and the current version is
working.

Use pyupgrade to convert simple string formatting to use f-string
syntax. pyupgrade is intentionally timid and will not create an f-string
if it would make the expression longer or if the substitution parameters
are anything but simple names or dotted names.

Unnecessary since dropping Python 2 support. In Python 3, all classes
are new style classes.

Unnecessary since dropping Python 2 support.

This makes one test case from test_str_to_display a duplicate and so has
been removed.

As we introduce stricter metadata parsing, we will need to ensure that
the wheel files used in our tests are compliant (except in the specific
way that we're testing against).

Currently we have a large number of test cases relying on undocumented or
under-documented wheel files that have various inconsistencies
(incorrect name, missing fields) that are unrelated to the features
actually under test.

With a new wheel builder helper function, we will be able to replace all
of our instances of pre-built wheel test fixtures with dynamically-generated
files in code that are correct by default.