What do you think of this idea for a slight modification to os.fspath:
the current version checks whether its arg is an instance of str, bytes
or any subclass and, if so, returns the arg unchanged. In all other
cases it tries to call the type's __fspath__ method to see if it can get
str, bytes, or a subclass thereof this way.

My proposal is to change this to:
1) check whether the type of the argument is str or bytes *exactly*; if
so, return the argument unchanged
2) check wether __fspath__ can be called on the type and returns an
instance of str, bytes, or any subclass (just like in the current version)
3) check whether the type is a subclass of str or bytes and, if so,
return it unchanged

This would have the following implications:
a) it would speed up the very common case when the arg is either a str
or a bytes instance exactly
b) user-defined classes that inherit from str or bytes could control
their path representation just like any other class
c) subclasses of str/bytes that don't define __fspath__ would still work
like they do now, but their processing would be slower
d) subclasses of str/bytes that accidentally define a __fspath__ method
would change their behavior

I think cases c) and d) could be sufficiently rare that the pros
outweigh the cons?