Add flake8 configuration

.flake8:
As flake8 can not yet be configured in a pyproject.toml, use a specific
file for it.
Set the maximum complexity to 10 (can even be lower) and add default
exclusions for project unrelated files.

.flake8

+[flake8]
+max-line-length = 120
+output-file = flake8.txt
+max-complexity = 10
+tee = True
+exclude =
+    .git
+    .tox
+    .venv
+    .cache
+    .eggs
+    .idea
+    .pytest*
+    *.egg-info
+    examples
+    doc
+extend-ignore=
+    # See https://github.com/PyCQA/pycodestyle/issues/373
+    E231,