Update plot_svm_regression.py

library imports have been moved to top of the file.
Now capable of treating sample_weights in feature space
FIX: Set correct impurity values in BestFirstBuilder
PEP8
Merge pull request #3049 from glouppe/tree-bestfirst

[MRG+1] Trees: set correct impurity values in BestFirstBuilder
target is response, not explanatory
typo: "fot" -> "for"
Merge pull request #3055 from ajschumacher/patch-3

typo: "fot" -> "for"
Merge pull request #3053 from ajschumacher/patch-1

target is response, not explanatory
FIX: weight impurity improvement by the size of the node
DOC: better doc for impurity_improvement
ENH: declare weighted_n_samples
ENH: rename Node attributes to avoid confusion
ENH: rename normalizer -> weighted_n_samples
ENH: renamed things too fast...
ENH: better API for passing weighted_n_samples
typo: "requiered" -> "required"
Merge pull request #3057 from ajschumacher/patch-4

typo: "requiered" -> "required"
DOC no input validation in constructors
ENH: Add dynamic maximum trial determination to RANSACRegressor