python3Packages.discordpy: fix libopus path on Darwin

Darwin uses .dylib as the extension for dynamic libraries (unlike Linux
which uses .so). Replace the hardcoded extension with
stdenv.hostPlatform.extensions.sharedLibrary to fix Opus support on
darwin.

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

@@ -1,4 +1,5 @@
 { lib
+, stdenv
 , aiohttp
 , buildPythonPackage
 , fetchFromGitHub
@@ -33,7 +34,7 @@ buildPythonPackage rec {
 
   patchPhase = ''
     substituteInPlace "discord/opus.py" \
-      --replace "ctypes.util.find_library('opus')" "'${libopus}/lib/libopus.so.0'"
+      --replace "ctypes.util.find_library('opus')" "'${libopus}/lib/libopus${stdenv.hostPlatform.extensions.sharedLibrary}'"
   '' + lib.optionalString withVoice ''
     substituteInPlace "discord/player.py" \
       --replace "executable='ffmpeg'" "executable='${ffmpeg}/bin/ffmpeg'"