adding a minimal .lgtm.yml file

This allows to specify the python version,
and to exclude some of the most common errors that are specific to sage

see https://trac.sagemath.org/ticket/28839

Author: fchapoton

.lgtm.yml

@@ -0,0 +1,8 @@
+queries:
+  - exclude: py/call/wrong-named-class-argument
+  - exclude: py/call/wrong-number-class-arguments
+  - exclude: py/unsafe-cyclic-import
+extraction:
+  python:
+    python_setup:
+      version: 3