DOC: removed duplicated Python 3 section

Fixes #2383.
DOC: docstring for predict_log_proba was wrong
Typo

Sorry for the noise :s
GBRT checks if ``loss`` is in self.supported_loss
renamed supported_loss to _SUPPORTED_LOSS (constants)
add test for deprecated warning
Merge pull request #2308 from pprett/gbrt-check-supported-loss

GBRT checks if ``loss`` is in self.supported_loss
COSMIT: fix docstrings in affinity_propagation
ENH: add fast_dot function
employ DataConversionWarning
FIX: avoid zero-dvision warnings
FIX: fix test warning
better handling of warning in test
COSMITS
FIX: workaround for missing BLAS
ENH: return np.dot if ndim does not match
CLEANUP
FIX: remove spurious test
ENH: add fast_dot shape check
ENH: cover another cornercase + fix messages
ENH: add new Warning class, improve tests, update docs