Removed deprecation warning in tracing example docs

[jcugat]

What do these changes do?

Fixes a documentation example to not emit a deprecation warning.

I've used asyncio.get_event_loop() instead of asyncio.get_running_loop() because the latter is only available in Python 3.7+.

Are there changes in behavior for the user?

No.

Related issue number

#3964

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-io]

Codecov Report

Merging #3965 into master will decrease coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #3965     +/-   ##
=========================================
- Coverage   97.77%   97.68%   -0.1%     
=========================================
  Files          43       43             
  Lines        8763     8763             
  Branches     1375     1375             
=========================================
- Hits         8568     8560      -8     
- Misses         83       90      +7     
- Partials      112      113      +1

Impacted Files	Coverage Δ
aiohttp/locks.py	92.85% <0%> (-7.15%)	⬇️
aiohttp/web_fileresponse.py	96.59% <0%> (-1.14%)	⬇️
aiohttp/streams.py	97.4% <0%> (-0.52%)	⬇️
aiohttp/helpers.py	97.04% <0%> (-0.5%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e76bd8a...9a6d483. Read the comment docs.

[webknjaz]

plz also go through the checklist

[jcugat]

I've added the news fragment, it's the only item in the checklist that applies.

[asvetlov]

Thanks!