don't iterate through names in __all__ which are not valid Python identifiers (PY-5503)

python/src/com/jetbrains/python/psi/impl/PyFileImpl.java

@@ -185,6 +185,9 @@ public class PyFileImpl extends PsiFileBase implements PyFile, PyExpression {
 
     if (remainingDunderAll != null) {
       for (String s: remainingDunderAll) {
+        if (!PyNames.isIdentifier(s)) {
+          continue;
+        }
         if (!processor.execute(new LightNamedElement(myManager, PythonLanguage.getInstance(), s), resolveState)) return false;
       }
     }

python/testData/completion/dunderAll/importSource.py

@@ -1,2 +1,2 @@
-__all__ = ['xyzzy', 'shazam']
+__all__ = ['xyzzy', 'shazam', 'xy is not a valid python identifier']