Increase minimum supported versions of Python and dependencies
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


For Maptlotlib 3.4 the :ref:`minimum supported versions
<install_requirements>` 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 :ref:`min_deps_policy` and `NEP29
<https://numpy.org/neps/nep-0029-deprecation_policy.html>`__