pyln.proto.message.*: Add Makefile to do mypy checks.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

contrib/pyln-proto/Makefile

@@ -1,10 +1,13 @@
 #! /usr/bin/make
 
-default: check-source check
-
 check:
 	pytest
 
-check-source:
+check-source: check-flake8 check-mypy
+
+check-flake8:
 	flake8 --ignore=E501,E731,W503
 
+# mypy . does not recurse.  I have no idea why...
+check-mypy:
+	mypy --ignore-missing-imports `find * -name '*.py'`