Fix new class detection

[petr-muller]

Previously, classes present in both versions of the file were reported
as new, which was caused by a buggy difference computation.

[coveralls]

Pull Request Test Coverage Report for Build 51

• 4 of 4 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 49:	0.0%
Covered Lines:	260
Relevant Lines:	260

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💛 - Coveralls

[codecov]

Codecov Report

Merging #9 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master     #9   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           5      5           
  Lines         257    260    +3     
=====================================
+ Hits          257    260    +3

Impacted Files	Coverage Δ
pyff/pyff.py	100% <100%> (ø)	⬆️

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