MNT: bump minimum python-dateutil version to 2.7

This was release in March 2018 and is the first version of
python-dateutil that does not have deprecation warnings due to the
collections -> collections.abc change in CPython.

Author: tacaswell

INSTALL.rst

@@ -147,7 +147,7 @@ Matplotlib requires the following dependencies:
 * `NumPy <https://numpy.org>`_ (>= 1.15)
 * `setuptools <https://setuptools.readthedocs.io/en/latest/>`_
 * `cycler <https://matplotlib.org/cycler/>`_ (>= 0.10.0)
-* `dateutil <https://pypi.org/project/python-dateutil>`_ (>= 2.1)
+* `dateutil <https://pypi.org/project/python-dateutil>`_ (>= 2.7)
 * `kiwisolver <https://github.com/nucleic/kiwi>`_ (>= 1.0.0)
 * `Pillow <https://pillow.readthedocs.io/en/latest/>`_ (>= 6.2)
 * `pyparsing <https://pypi.org/project/pyparsing/>`_ (>=2.0.3)

requirements/testing/travis36minver.txt

@@ -1,6 +1,6 @@
-# Extra pip requirements for the first travis python 3.6 build
+# Extra pip requirements for the minimum-version travis run
 
 cycler==0.10
-python-dateutil==2.1
+python-dateutil==2.7
 numpy==1.15.0
 pyparsing==2.0.3

setup.py

@@ -283,7 +283,7 @@ def build_extensions(self):
         "numpy>=1.15",
         "pillow>=6.2.0",
         "pyparsing>=2.0.3,!=2.0.4,!=2.1.2,!=2.1.6",
-        "python-dateutil>=2.1",
+        "python-dateutil>=2.7",
     ],
 
     cmdclass=cmdclass,