Merge pull request matplotlib#20454 from QuLogic/lgtm-py3

Tell LGTM to use Python 3 explicitly.

Author: timhoffm

.lgtm.yml

@@ -1,14 +1,18 @@
+extraction:
+  python:
+    python_setup:
+      version: 3
 path_classifiers:
-    examples:
-        # Exclude example files from analysis by tagging them.
-        # We intentionally use multiple imports and unused variables in the
-        # examples to make them more explicit. These generate a lot of alerts.
-        # Since lgtm does not yet support suppressing selected alerts per
-        # file, we currently deactivate analysis completely here. May be
-        # reactivated when we can selectively suppress
-        # - py/import-and-import-from
-        # - py/repeated-import
-        # - py/unused-local-variable
-        # - py/multiple-definition
-        - examples
-        - tutorials
+  examples:
+    # Exclude example files from analysis by tagging them.
+    # We intentionally use multiple imports and unused variables in the
+    # examples to make them more explicit. These generate a lot of alerts.
+    # Since lgtm does not yet support suppressing selected alerts per
+    # file, we currently deactivate analysis completely here. May be
+    # reactivated when we can selectively suppress
+    # - py/import-and-import-from
+    # - py/repeated-import
+    # - py/unused-local-variable
+    # - py/multiple-definition
+    - examples
+    - tutorials