FIX: pep8 and Py3 support in sklearn.neighbors.base
FIX: Python 3 support for the neighbors doctests
FIX: pep8 + Py3 fixes in test_dist_metrics
FIX: pep8 and Py3 support in sklearn.neighbors.dist_metrics
FIX: Py3 / pep8 fixes in test_ball_tree / test_kd_tree
Update Python 3 support status
FIX use float64 in metrics.r2_score() to prevent overflow

Without this, if the input arrays are of type np.float32, their sums
may be computed with an large accumulated error, resulting in the wrong
scor with very long arrays (millions of elements).

The "1 - numerator / denominator" calculation at the very end produces
a float64 anyway, so the returned type does not change--only the accuracy.

Fixes #2158.
TST r2_score float32 overflow fix
Revert "TST r2_score float32 overflow fix"

This reverts commit 1b9ad774ac8ddfabc6c72d2cf7568c2d30342fbb.

Test doesn't work consistently. Functionality left in place.