Add backtick to remove_quotes character list

[codenamelxl]

Fix #628 Consider backtick `as a quote character and remove it in utils.removte_quotes()

[codecov[bot]]

Codecov Report

Merging #629 (797792c) into master (23d2993) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #629   +/-   ##
=======================================
  Coverage   96.82%   96.82%           
=======================================
  Files          20       20           
  Lines        1514     1514           
=======================================
  Hits         1466     1466           
  Misses         48       48           

Impacted Files	Coverage Δ
sqlparse/utils.py	98.24% <100.00%> (ø)

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[andialbrecht]

Thanks!