Added deprecation warning for add_link() method

[abidahmadq]

Summary of Changes

Added deprecation warning to the add_link() method.

Related Issues

Related to issue https://github.com/falconry/falcon/issues/1803

Pull Request Checklist

• [x] Applied changes to both WSGI and ASGI code paths and interfaces (where applicable).
• [x] Added tests for changed code.
• [x] Prefixed code comments with GitHub nick and an appropriate prefix.
• [x] Coding style is consistent with the rest of the framework.
• [x] Updated documentation for changed code.
  • [x] Added docstrings for any new classes, functions, or modules.
  • [x] Updated docstrings for any modifications to existing code.
  • [x] Updated both WSGI and ASGI docs (where applicable).
  • [x] Added references to new classes, functions, or modules to the relevant RST file under docs/.
  • [x] Updated all relevant supporting documentation files under docs/.
  • [x] A copyright notice is included at the top of any new modules (using your own name or the name of your organization).
  • [x] Changed/added classes/methods/functions have appropriate versionadded, versionchanged, or deprecated directives.
• [x] Changes (and possible deprecations) have towncrier news fragments under docs/_newsfragments/, with the file name format {issue_number}.{fragment_type}.rst. (Run towncrier --draft to ensure it renders correctly.)

[abidahmadq]

@vytas7 please take a look.

[codecov[bot]]

Codecov Report

Merging #1928 (cc92edb) into master (ddb4a03) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #1928   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           63        63           
  Lines         6676      6677    +1     
  Branches      1239      1239           
=========================================
+ Hits          6676      6677    +1     

Impacted Files	Coverage Δ
falcon/response.py	100.00% <100.00%> (ø)
falcon/util/deprecation.py	100.00% <100.00%> (ø)

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[abidahmadq]

@vytas7 what should i do about the deprecation warning?
`class LinkHeaderResource: def init(self): self._links = []

def append_link(self, *args, **kwargs):
    self._links.append((args, kwargs))

def on_get(self, req, resp):
    resp.text = '{}'

    append_link = None
    for args, kwargs in self._links:
        append_link = (
            resp.append_link if append_link is resp.add_link else resp.add_link  
        )
        append_link(*args, **kwargs) --> line which is emiting the error. 

`

How should i fix it? 1) remove the resp.add_link from the above line so it doesn't use the Alias. 2) Use @pytest.mark.filterwarnings to ignore the warnings. 3) Use with pytest.warns(UserWarning): to catch it.

[CaselIT]

I think you can use the same pattern as here: https://github.com/falconry/falcon/blob/fb0d68688b29ae518f98798c278a5815e40660ca/tests/test_response_media.py#L182-L189

[vytas7]

Hi again @abidahmadq , just checking if you had a chance to work on the second batch of my comments?

[abidahmadq]

@vytas7 finding it confusing on how to proceed with this. Will take a look again on this weekend.

[abidahmadq]

Thanks @vytas7