4.2.0

Dropped support for Pythons 2.7 and 3.5: PR #525. From now on, Uproot only supports Python 3.6 and up.

@henryiii dropped Python2isms from the codebase: PR #526.

@jrueb made writing to Python file handles work (fixed a half-finished, forgotten implementation): PR #538.

@jpivarski fixed cut jagged-array corner-case in library="pd". Fixed a case in which the instance version is 0, but the streamer version is not: PR #537. Fixed uproot.WritableTTree.extend when the metadata needs to be rewritten: PR #547. When checking to see if something in file["name"] = something is an Awkward Array or Pandas DataFrame for creating a TTree, also check for superclasses (the whole mro): PR #557.