src/sage/doctest/parsing.py: Do not fail if RealIntervalField cannot …

…be imported, just warn

src/sage/doctest/parsing.py

@@ -83,9 +83,19 @@ def RIFtol(*args):
     """
     global _RIFtol
     if _RIFtol is None:
-        # We need to import from sage.all to avoid circular imports.
-        from sage.all import RealIntervalField
-        _RIFtol = RealIntervalField(1044)
+        try:
+            # We need to import from sage.all to avoid circular imports.
+            from sage.all import RealIntervalField
+        except ImportError:
+            from warnings import warn
+            warn("RealIntervalField not available, ignoring all tolerance specifications in doctests")
+            def fake_RIFtol(*args):
+                if len(args) == 2:
+                    return (args[0] + args[1]) / 2
+                return args[0]
+            _RIFtol = fake_RIFtol
+        else:
+            _RIFtol = RealIntervalField(1044)
     return _RIFtol(*args)
 
 # This is the correct pattern to match ISO/IEC 6429 ANSI escape sequences: