Avoid tokens copy

[living180]

Partial fix for #621:

In TokenList._token_matching() avoid quadratic behavior resulting from taking a slice of self.tokens during each invocation. Since we are working with indexes anyway, just generate the indexes using range() and use normal indexing to access the desired tokens instead of calling enumerate() with a slice from self.tokens.

[codecov[bot]]

Codecov Report

Merging #622 (b4fb5de) into master (23d2993) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #622   +/-   ##
=======================================
  Coverage   96.82%   96.82%           
=======================================
  Files          20       20           
  Lines        1514     1514           
=======================================
  Hits         1466     1466           
  Misses         48       48           

Impacted Files	Coverage Δ
sqlparse/sql.py	97.60% <100.00%> (ø)

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[andialbrecht]

Nice refactoring, thank you!