Add missing assert (#167)

* Add missing assert

* Fix docstring test for Python 3.13

* Fix older Python version

Author: AdrienVannson

betterproto2/tests/test_documentation.py

@@ -1,5 +1,6 @@
 import ast
 import inspect
+import sys
 
 from tests.util import requires_grpclib  # noqa: F401
 
@@ -43,7 +44,19 @@ def test_documentation(requires_grpclib) -> None:
 def test_escaping(requires_grpclib) -> None:
     from .outputs.documentation.documentation import ComplexDocumentation
 
-    ComplexDocumentation.__doc__ == """
+    if sys.version_info >= (3, 13):
+        assert (
+            ComplexDocumentation.__doc__
+            == """
+A comment with backslashes \\ and triple quotes \"\"\"
+Simple quotes are not escaped "
+"""
+        )
+    else:
+        assert (
+            ComplexDocumentation.__doc__
+            == """
     A comment with backslashes \\ and triple quotes \"\"\"
     Simple quotes are not escaped "
     """
+        )