Remove external test dependency to http://httpbin.org

[derlih]

What do these changes do?

Remove external test dependency to http://httpbin.org

Are there changes in behavior for the user?

No

Related issue number

#5840

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• If you provide code modification, please add yourself to CONTRIBUTORS.txt
  • The format is <Name> <Surname>.
  • Please keep alphabetical order, the file is sorted by names.
• Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> for example (588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the pr
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: "Fix issue with non-ascii contents in doctest text files."

[codecov]

Codecov Report

Merging #5843 (e40e1a9) into master (816e0bc) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #5843   +/-   ##
=======================================
  Coverage   93.31%   93.31%           
=======================================
  Files         102      102           
  Lines       30063    30066    +3     
  Branches     2690     2689    -1     
=======================================
+ Hits        28054    28057    +3     
  Misses       1833     1833           
  Partials      176      176           

Flag	Coverage Δ
unit	93.24% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
tests/test_formdata.py	100.00% <100.00%> (ø)

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[patchback]

Backport to 3.8: 💚 backport PR created

✅ Backport PR branch: patchback/backports/3.8/728505c119e6abe96968f13567aaa7e4cbcdbe8f/pr-5843

Backported as #6112

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