Applying this patch - I tried this hypothetical test.

$ ./python.exe -m timeit -s "import urllib.parse; x='a%20' * 100000"
"urllib.parse.unquote(x)"
10 loops, best of 3: 205 msec per loop

Without the patch, the above test will run for minutes!

This creates a significant difference for extremely long query
strings. For small strings the performance is negligible.

Serhiy: One question. Is there a need to have  -

+ append = res.append

And then use 'append'? res.append is easier for readability IMO.
Besides that I dont have any other comments. Maciej's comment could be
sought as he (correctly) brought up this issue.