Remove the egg file when cleaning, fix tests by adding PYTHONPATH

debian/clean

 debian/diffoscope.1
 debian/diffoscope.bash-completion
+diffoscope.egg-info/

debian/rules

@@ -52,7 +52,7 @@ override_dh_auto_test:
 # Let's avoid plonking everything in the "DOS/MBR" category, this is really not
 # a sustainable filetype testing solution (e.g. c051ba9d and c1442405).
 	! grep 'DOS/MBR' -R diffoscope
-	dh_auto_test -O--buildsystem=pybuild
+	PYTHONPATH="$(CURDIR)" dh_auto_test -O--buildsystem=pybuild
 
 override_dh_installman:
 	dh_installman -O--buildsystem=pybuild