pyproject.toml: update mypy configuration

pyproject.toml

@@ -1,6 +1,9 @@
 [tool.mypy]
+cache_dir = "build/.mypy_cache"
 explicit_package_bases = true
 mypy_path = "$MYPY_CONFIG_FILE_DIR/src/sst/core/testingframework"
+# This should be 3.6 but is not supported with the newest versions of mypy.
+python_version = "3.8"
 
 warn_unused_ignores = true
 
@@ -15,5 +18,10 @@ exclude = [
 ]
 
 [[tool.mypy.overrides]]
-module = "sst"
+module = [
+    "blessings",
+    "sst",
+    "testtools",
+    "testtools.testsuite",
+]
 ignore_missing_imports = true