4.1.6

@duncanmmacleod included the test suite in the source distribution: PR #477.

@btovar fixed exception handling in XRootD callback threads by passing them on to the main thread: PR #480.

@jpivarski restricted Uproot 4.x to Awkward 1.x: PR #478. Also fixed some bugs when writing TTrees to preexisting files ("update" mode): PR #488.