Add coverage.py invocation to Makefile

Signed-off-by: martin f. krafft <madduck@madduck.net>
diff --git a/.gitignore b/.gitignore
index 18f6e5b..b1c78c9 100644
--- a/.gitignore
+++ b/.gitignore
@@ -5,3 +5,4 @@
 /build
 /dist
 *.egg-info
+/.coverage