Enable flake8 testing with pytest

Mask E402 in two files until Python2 is dropped

setup.cfg

@@ -0,0 +1,17 @@
+# The ignore list for flake8 itself when run on the command line is distinct
+# from the ignore list used by the pytest-flake8 plugin. This provides more
+# control over testing but does require that the lists are kept in sync
+# if there are later changes to the developer guide standard.
+[flake8]
+max-line-length = 110
+ignore = E133, E226, E228, N802, N803, N806
+exclude = __init__.py, testLib.py, ticket1914.py
+
+[tool:pytest]
+addopts = --flake8
+flake8-ignore = E133 E226 E228 N802 N803 N806
+    # These can be fixed when python2 is dropped
+    # ie E402 module level import not at top of file
+    # These are caused by storing `str` before imports override it
+    python/lsst/pex/config/choiceField.py E402
+    python/lsst/pex/config/config.py E402

tests/SConscript

@@ -2,4 +2,4 @@
 from lsst.sconsUtils import scripts
 scripts.BasicSConscript.pybind11(['testLib'])
 scripts.BasicSConscript.tests(noBuildList=['testLib.cc'],
-                              ignoreList=['testLib.py'])
+                              pyList=[])