Don't flag B009/B010 if identifiers would be mangled (#2204)

src/python/identifiers.rs

@@ -13,3 +13,12 @@ pub fn is_identifier(s: &str) -> bool {
     // Are the rest of the characters letters, digits, or underscores?
     s.chars().skip(1).all(|c| c.is_alphanumeric() || c == '_')
 }
+
+/// Returns `true` if a string is a private identifier, such that, when the
+/// identifier is defined in a class definition, it will be mangled prior to
+/// code generation.
+///
+/// See: <https://docs.python.org/3.5/reference/expressions.html?highlight=mangling#index-5>.
+pub fn is_mangled_private(id: &str) -> bool {
+    id.starts_with("__") && !id.ends_with("__")
+}