Fix relative file url repr

[q0w]

What do these changes do?

Are there changes in behavior for the user?

Related issue number

Closes #648

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> (e.g. 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 #705 (f0b6ee3) into master (1a43a04) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #705   +/-   ##
=======================================
  Coverage   99.73%   99.73%           
=======================================
  Files           4        4           
  Lines         750      755    +5     
  Branches      170      171    +1     
=======================================
+ Hits          748      753    +5     
  Misses          2        2           

Flag	Coverage Δ
unit	99.60% <100.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
yarl/_url.py	99.65% <100.00%> (+<0.01%)	⬆️

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