ci: update imgtool script to to use python/pip

Switch from hard-coded python3/pip to base python/pip. Also install full
imgtool package.

Signed-off-by: Fabio Utzig <utzig@apache.org>
Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/ci/imgtool_run.sh b/ci/imgtool_run.sh
index 20b5420..7358706 100755
--- a/ci/imgtool_run.sh
+++ b/ci/imgtool_run.sh
@@ -30,10 +30,10 @@
 
 cd scripts/
 
-last_release=$(pip3 show imgtool | grep "Version: " | cut -d" " -f2)
+last_release=$(pip show imgtool | grep "Version: " | cut -d" " -f2)
 repo_version=$(grep "imgtool_version = " imgtool/__init__.py | sed 's/^.* = "\(.*\)"/\1/g')
 
-python3 ../ci/compare_versions.py --old $last_release --new $repo_version
+python ../ci/compare_versions.py --old $last_release --new $repo_version
 rc=$?
 
 if [[ $rc -eq 0 ]]; then
@@ -48,6 +48,6 @@
 fi
 
 rm -rf $DIST_DIR
-python3 setup.py sdist bdist_wheel
+python setup.py sdist bdist_wheel
 
 twine upload --username __token__ --password "${TWINE_TOKEN}" "${DIST_DIR}/*"