Add support for Python 3.9 and official support for PySide2

• remove dependency on future to reduce import time PR #255
• add constraints to enforce image aspect ratio in ImageView PR #254
• improvements to the scintilla widget and live editor PR #243

• fix import of QScintilla under PyQt5 PR #230
• add support for importing enaml files from zip archives #232
• fix menu item ordering under Python 3 #233
• fix repr of declarative function objects #235
• properly reset layout_container member in qt backend after a widget has been
  reparented #240
• fix calls to explicit_warn which could lead to global vars corruption #247
• add a text align member to Field to control text alignment #249
• fix the parsing rules for function definitions #252
• several improvements to the handling of comprehensions and lambdas #242 #245

A error in the overridden Install command was preventing the proper building of conda packages.

This release bring support for Python 3.3, 3.4, 3.5 and 3.6 (with the exception of f-strings in enaml files) and support for Qt5 through the use of the QtPy package. The Qt modules have hence been partially refactored to follow the Qt5 layout which may cause backward compatibility issues.