Run tests in Python development mode

Can reveal additional issues in the library, and helps understanding
failures by providing more context.
https://docs.python.org/3/library/devmode.html#devmode

Makefile

@@ -54,6 +54,7 @@ testall:
 test:
  python \
  -b \
+ -X dev \
  -Werror \
  -Wdefault:"the imp module is deprecated in favour of importlib; see the module's documentation for alternative uses":DeprecationWarning:distutils: \
  -Wdefault:"Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working":DeprecationWarning:: \