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Lib/quopri.py for version 3.3..3.5-tip contains following code:
ESCAPE = b'='
...
 line = input.readline()
 if not line: break
 i, n = 0, len(line)
 if n > 0 and line[n-1:n] == b'\n':
...
 elif i+1 < n and line[i+1] == ESCAPE:
So, ESCAPE is defined as bytes, we see that "line" is read as bytes, and characters are accessed using idiom like "line[n-1:n]", but then it uses "line[i+1]", which returns int and thus will never be equal to ESCAPE.
I'm not sure what exact semantic condition that branch represents, for me it looks like "==" sequence to be decoded as "=". But I don't see such encoding variant in http://en.wikipedia.org/wiki/Quoted-printable . Either way, replacing that condition with "and False", still passes test_quopri.py