Keep a trailing slash in URL.joinpath

[gmacon]

What do these changes do?

This fixes the regression in URL.joinpath that stopped it from preserving a trailing slash in the path components to be joined.

Are there changes in behavior for the user?

A statement like URL("http://example.com") / "foo/" will again preserve the trailing slash.

Related issue number

Fixes #862

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 #866 (bd789e0) into master (cfc634e) will decrease coverage by 0.65%.
The diff coverage is 100.00%.

❗ Current head bd789e0 differs from pull request most recent head c23432a. Consider uploading reports for the commit c23432a to get more accurate results

@@            Coverage Diff             @@
##           master     #866      +/-   ##
==========================================
- Coverage   99.74%   99.09%   -0.65%     
==========================================
  Files           4        4              
  Lines         772      775       +3     
  Branches      219      178      -41     
==========================================
- Hits          770      768       -2     
- Misses          2        4       +2     
- Partials        0        3       +3     

Flag	Coverage Δ
unit	99.09% <100.00%> (-0.52%)	⬇️

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Impacted Files	Coverage Δ
yarl/_url.py	99.67% <100.00%> (+<0.01%)	⬆️

... and 2 files with indirect coverage changes

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[mjpieters]

Excellent, this looks great.

I've requested a small change that simplifies the _make_child() logic a little; once that's done this should be good to go.