Fix some errors in unquoter

[serhiy-storchaka]

• No longer raise ValueError for % followed by non-hexdigits.
• Fix decoding % followed by a space and hexdigit.

Closes #516.
Closes #520.

[codecov]

Codecov Report

Merging #521 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #521   +/-   ##
=======================================
  Coverage   99.72%   99.72%           
=======================================
  Files           4        4           
  Lines         739      739           
  Branches      168      167    -1     
=======================================
  Hits          737      737           
  Misses          2        2           

Impacted Files	Coverage Δ
yarl/_quoting_py.py	100.00% <100.00%> (ø)

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[lgtm-com]

This pull request fixes 1 alert when merging 8c432d3 into 5967c55 - view on LGTM.com

fixed alerts:

• 1 for Nested loops with same variable

[lgtm-com]

This pull request fixes 1 alert when merging 7d81968 into 7add4d4 - view on LGTM.com

fixed alerts:

• 1 for Nested loops with same variable

[asvetlov]

Thanks!