d/rules: Update build instruction

debian/rules

 #!/usr/bin/make -f
 
 export PYBUILD_NAME=swh.model
-export PYBUILD_TEST_ARGS=--with-doctest -sv -a !db,!fs
+export export PYBUILD_TEST_ARGS=-m 'not db and not fs'
 
 %:
 	dh $@ --with python3 --buildsystem=pybuild