require 3.x.x releases of platformdirs

we don't want to suddenly/unexpectedly break stuff for borg users
just because platformdirs does a breaking release.

at platformdirs 2.0.0 macOS config dir changed.
at platformdirs 3.0.0 macOS config dir changed again.
at platformdirs 4.0.0 (future) - who knows?

setup.cfg

@@ -41,7 +41,7 @@ setup_requires =
 install_requires =
   msgpack >=1.0.3, <=1.0.4
   packaging
-  platformdirs
+  platformdirs >=3.0.0, <4.0.0
   argon2-cffi
 tests_require =
   pytest