Merge remote-tracking branch 'upstream/stable'

Conflicts:
	Orange/widgets/data/owcontinuize.py
	Orange/widgets/data/owfile.py
	Orange/widgets/data/owmergedata.py
	Orange/widgets/data/owpaintdata.py
	requirements-core.txt

Release 3.3.9

Release 3.3.8

Release 3.3.7

Release 3.3.6

Release 3.3.5

Release 3.3.4

Release 3.3.3

Release 3.3.2

Merge pull request biolab#1135 from ales-erjavec/win-install-update-pip

Windows installer: update pip on target system if required