Tweaking warnings related to missing accessibility fields text and fallback

[filmaj]

This follows the pattern in the solution converged on for slackapi/node-slack-sdk#1476.

Wondering if there is an opportunity to DRY the tests up here, especially across the legacy vs. the regular synchronous client code. The tests seem to be a carbon copy. I don't really have a suggestion on how to improve that though, but maybe some more Python-y folks have ideas?

[codecov]

Codecov Report

Merging #1208 (d26a0b3) into main (8a4f44b) will increase coverage by 0.00%.
The diff coverage is 95.83%.

@@           Coverage Diff           @@
##             main    #1208   +/-   ##
=======================================
  Coverage   86.55%   86.55%           
=======================================
  Files         111      111           
  Lines       10893    10891    -2     
=======================================
- Hits         9428     9427    -1     
+ Misses       1465     1464    -1     

Impacted Files	Coverage Δ
slack_sdk/web/internal_utils.py	94.44% <91.66%> (-0.09%)	⬇️
slack_sdk/web/async_client.py	85.10% <100.00%> (ø)
slack_sdk/web/client.py	86.48% <100.00%> (ø)
slack_sdk/web/legacy_client.py	86.20% <100.00%> (ø)
slack_sdk/socket_mode/builtin/internals.py	72.80% <0.00%> (+0.43%)	⬆️

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[seratch]

LGTM -As for the future, having mostly the same tests for WebClient and AsyncWebClient should be fine. As for the legacy client, we won't add tests for future changes. Only in the early days of slack-sdk v3 development, we tended to have tests for both legacy and current clients.