In Python 3, the default encoding of source files is utf-8. The encoding
cookie is now unnecessary and redundant so remove it. For more details,
see the docs:

https://docs.python.org/3/howto/unicode.html#the-string-type

> The default encoding for Python source code is UTF-8, so you can
> simply include a Unicode character in a string literal ...

Includes a fix for the flake8 header checks to stop expecting an
encoding cookie.

Remove unnecessary bytes type check in ModuleAnalyzer.for_string()

In Python 3, both .encode() and .decode() default the encoding to
'utf-8'. See the docs:

https://docs.python.org/3/library/stdtypes.html#str.encode
https://docs.python.org/3/library/stdtypes.html#bytes.decode

Simplify and shorten the code by using the default instead of
respecifying it.

All uses always pass a str, never a bytes. Per the type signature, only
str types are allowed.