fix: Slack log service (#2537)

### Description

<!--Add a description of your PR here-->

This updates the Slack ``--log-service``  feature added in #170.
The ``Slacker`` package is no longer maintained, and legacy slack tokens
are no longer generatable.
I've replace it with the officially supported [Slack
SDK](https://github.com/slackapi/python-slack-sdk) (see ``setup.cfg``),
preserving the original functionality with minimal adjustments.

This has been minimally tested for functionality. However, the original
implementation has a couple issues (which this PR **does not** address)
-

1. Lack of `docs/` on how to set up the ``--log-service slack`` feature
via Slack
2. Logic is not robust - e.g. when `msg["done"] == msg["total"]`, this
does not mean the workflow is complete (checkpoints)
3. The implementation of ``--log-service slack`` is synchronous, meaning
that Snakemake workflows will wait for the implemented `log_handler`
function to finish. For normal print statements, this isn't an issue,
but repeated calls of Slack WebAPI's `postMessage` can significantly
slow things down. Depending on the workflow (Snakefile), this can become
noticeable.
4. The implementation does not comply with Slack WebAPI's rate limiting.

The current implementation will look like this - messages on slack are
sent by the user, using a user OAuth token.

![image](https://github.com/snakemake/snakemake/assets/43220811/69e3b335-13fc-4af1-8538-8d65920f9494)

I've already addressed points 2, 3, and 4 in a private repository using
``--log-handler`` that should be integrable into this current
implementation, but it has not been robustly tested (integration can
happen at a later PR).

![image](https://github.com/snakemake/snakemake/assets/43220811/97576f52-1c46-4237-806a-e823bd76f9f7)

Thanks!

### QC
<!-- Make sure that you can tick the boxes below. -->

* [ ] The PR contains a test case for the changes or the changes are
already covered by an existing test case.
* [ ] The documentation (`docs/`) is updated to reflect the changes or
this is not necessary (e.g. if the change does neither modify the
language nor the behavior or functionalities of Snakemake).

---------

Co-authored-by: Johannes Köster <johannes.koester@tu-dortmund.de>

setup.cfg

@@ -63,7 +63,7 @@ install_requires =
     conda-inject >=1.3.1,<2.0
 
 [options.extras_require]
-messaging = slacker
+messaging = slack_sdk
 pep =
     eido
     peppy

snakemake/logging.py

@@ -86,38 +86,39 @@ def decorate(self, record):
 
 class SlackLogger:
     def __init__(self):
-        from slacker import Slacker
+        from slack_sdk import WebClient
 
         self.token = os.getenv("SLACK_TOKEN")
         if not self.token:
             print(
-                "The use of slack logging requires the user to set a user specific slack legacy token to the SLACK_TOKEN environment variable. Set this variable by 'export SLACK_TOKEN=your_token'. To generate your token please visit https://api.slack.com/custom-integrations/legacy-tokens."
+                "The use of slack logging requires the user to set a user specific slack User OAuth token to the SLACK_TOKEN environment variable. Set this variable by 'export SLACK_TOKEN=your_token'. To generate your token please visit https://api.slack.com/authentication/token-types#user."
             )
             exit(-1)
-        self.slack = Slacker(self.token)
+        self.slack = WebClient(self.token)
         # Check for success
         try:
-            auth = self.slack.auth.test().body
+            auth = self.slack.auth_test().data
         except Exception:
             print(
-                "Slack connection failed. Please compare your provided slack token exported in the SLACK_TOKEN environment variable with your online token at https://api.slack.com/custom-integrations/legacy-tokens. A different token can be set up by 'export SLACK_TOKEN=your_token'."
+                "Slack connection failed. Please compare your provided slack token exported in the SLACK_TOKEN environment variable with your online token (app). This token can be tested at https://api.slack.com/methods/auth.test/test. A different token can be set up by 'export SLACK_TOKEN=your_token'."
             )
             exit(-1)
         self.own_id = auth["user_id"]
         self.error_occured = False
+        self.slack.chat_postMessage(
+            channel=self.own_id, text="Snakemake has connected."
+        )
 
     def log_handler(self, msg):
-        if msg["level"] == "error" and not self.error_occured:
-            self.slack.chat.post_message(
-                self.own_id, text="At least one error occured.", username="snakemake"
+        if "error" in msg["level"] and not self.error_occured:
+            self.slack.chat_postMessage(
+                channel=self.own_id, text="At least one error occured."
             )
             self.error_occured = True
 
         if msg["level"] == "progress" and msg["done"] == msg["total"]:
             # workflow finished
-            self.slack.chat.post_message(
-                self.own_id, text="Workflow complete.", username="snakemake"
-            )
+            self.slack.chat_postMessage(channel=self.own_id, text="Workflow complete.")
 
 
 class WMSLogger: