Remove use of unnecessary compat shim, six.binary_type

The type `bytes` is available on all supported Python versions. On
Python 2.7, it is an alias of str, same as six.binary_type.

Makes the code slightly more forward compatible.

changelog/4248.trivial.txt

@@ -0,0 +1 @@
+Remove use of unnecessary compat shim, six.binary_type

src/_pytest/pathlib.py

@@ -159,7 +159,7 @@ def create_cleanup_lock(p):
     else:
         pid = os.getpid()
         spid = str(pid)
-        if not isinstance(spid, six.binary_type):
+        if not isinstance(spid, bytes):
             spid = spid.encode("ascii")
         os.write(fd, spid)
         os.close(fd)