Configure flake8 to work with black

Using black's default line length of 88.

Black follows the PEP8 standard more closely than
pycodestyle does (as used in the flake8 checks)
so must disable E203 until this is fixed:
PyCQA/pycodestyle#373

Author: peterjc

.flake8

@@ -0,0 +1,6 @@
+[flake8]
+doctests = True
+max-line-length = 88
+extend-ignore =
+    # See https://github.com/PyCQA/pycodestyle/issues/373
+    E203,