Increase minimum supported versions of Python and dependenciesΒΆ

For Maptlotlib 3.4 the minimum supported versions are being bumped

Dependency min in mpl3.3 min in mpl3.4
Python 3.6 3.7
dateutil 2.1 2.2.1
numpy 1.15 1.16
pyparsing 2.0.3 2.2.1

This consistent with our Minimum Version of Dependencies Policy and NEP29