Fix incorrect usage of os.path.sep in rewrite.py

os.path.sep is not an official API, and is probably result of it being imported (and so available), however it seems that is not the case on all systems,

I git blamed this on me, and I'm sure it was an accident (I probably meant `os.sep.join`), but let's go with `os.path.join` as that's the sane thing to do.

Fix pytest-dev#9791

changelog/9791.bugfix.rst

@@ -0,0 +1 @@
+Fixed a path handling code in ``rewrite.py`` that seems to work fine, but was incorrect and fails in some systems.

src/_pytest/assertion/rewrite.py

@@ -190,7 +190,7 @@ def _early_rewrite_bailout(self, name: str, state: "AssertionState") -> bool:
             return False
 
         # For matching the name it must be as if it was a filename.
-        path = PurePath(os.path.sep.join(parts) + ".py")
+        path = PurePath(os.path.join(*parts) + ".py")
 
         for pat in self.fnpats:
             # if the pattern contains subdirectories ("tests/**.py" for example) we can't bail out based