python3Packages.python-olm: remove dependency on the typing package

Typing has been upstreamed into CPython and will cause errors
if loaded with a 3.7 interpreter. Cf.
https://github.com/python/typing/issues/573

Under Python3 libolm works without typing as tested via
weechat-matrix.

typing is still pulled in when installed for Python2.

pkgs/development/python-modules/python-olm/default.nix

@@ -1,5 +1,5 @@
 { lib, buildPythonPackage, olm,
-  cffi, future, typing }:
+  cffi, future, isPy3k, typing }:
 
 buildPythonPackage {
   pname = "python-olm";
@@ -15,8 +15,7 @@ buildPythonPackage {
   propagatedBuildInputs = [
     cffi
     future
-    typing
-  ];
+  ] ++ lib.optionals (!isPy3k) [ typing ];
 
   doCheck = false;