Simplify the hex formatting in _url.py

[alexwlchan]

Following up on my suggestion in #23. Makes this line a little more readable, and consistent with line 177:

ret[c] = ret[v] = '%{0:02X}'.format(i)

Informal testing with timeit is inconclusive, suggesting they’re about the same time.

[codecov-io]

Codecov Report

Merging #29 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #29   +/-   ##
=======================================
  Coverage   96.78%   96.78%           
=======================================
  Files           6        6           
  Lines        1026     1026           
  Branches      123      123           
=======================================
  Hits          993      993           
  Misses         19       19           
  Partials       14       14

Impacted Files	Coverage Δ
hyperlink/_url.py	93.69% <100%> (ø)	⬆️

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[mahmoud]

lgtm!