python3Packages.slackclient: fix pname

The python package is called slackclient, as can be seen in setup.py for
this version.

https://github.com/slackapi/python-slack-sdk/blob/v2.9.3/setup.py?rgh-link-date=2021-04-23T00%3A03%3A28Z#L217

This prevented home-assistant tooling from finding the package and
providing support for the component.

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

···
 }:
 
 buildPythonPackage rec {
-  pname = "python-slackclient";
+  pname = "slackclient";
   version = "2.9.3";
 
   disabled = !isPy3k;

pkgs/servers/home-assistant/component-packages.nix

···
     "sky_hub" = ps: with ps; [ ]; # missing inputs: pyskyqhub
     "skybeacon" = ps: with ps; [ ]; # missing inputs: pygatt[GATTTOOL]
     "skybell" = ps: with ps; [ skybellpy ];
-    "slack" = ps: with ps; [ ]; # missing inputs: slackclient
+    "slack" = ps: with ps; [ slackclient ];
     "sleepiq" = ps: with ps; [ sleepyq ];
     "slide" = ps: with ps; [ ]; # missing inputs: goslide-api
     "sma" = ps: with ps; [ pysma ];