Use $PYTHON when running mypy

Make sure to run mypy with the desired Python version.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/tests/scripts/check-python-files.sh b/tests/scripts/check-python-files.sh
index 322e92b..5796c29 100755
--- a/tests/scripts/check-python-files.sh
+++ b/tests/scripts/check-python-files.sh
@@ -53,7 +53,7 @@
     # minimum version is required. The check is not just "type mypy"
     # because that passes if a mypy exists but is not installed for the current
     # python version.
-    mypy --version 2>/dev/null >/dev/null
+    $PYTHON -m mypy --version 2>/dev/null >/dev/null
 }
 
 # With just a --can-xxx option, check whether the tool for xxx is available
@@ -77,7 +77,7 @@
 if can_mypy; then
     echo
     echo 'Running mypy ...'
-    mypy scripts/*.py tests/scripts/*.py ||
+    $PYTHON -m mypy scripts/*.py tests/scripts/*.py ||
       ret=1
 fi