Fix invalid syntax deprecation warning in regex

[amureki]

Solves

pytest_translations/po_spelling.py:82
  pytest-translations/pytest_translations/po_spelling.py:82: DeprecationWarning: invalid escape sequence '\%'
    text = re.sub('\%\(.*?\)', '', text)

[codecov]

Codecov Report

Merging #52 (bfb8322) into master (8d7661f) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #52   +/-   ##
=======================================
  Coverage   94.14%   94.14%           
=======================================
  Files           6        6           
  Lines         188      188           
=======================================
  Hits          177      177           
  Misses         11       11           

Impacted Files	Coverage Δ
pytest_translations/po_spelling.py	89.47% <100.00%> (ø)

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