Remove tests that fail under CPython 3.5

See http://bugs.python.org/issue19995 for rationale. As micropython currently
aims for Python 3.3 compatibility I have not changed behaviour, but this
change allows the test suite to be run against a newer Python without having
spurious failures.

tests/float/string-format-modulo.py

@@ -4,9 +4,6 @@ print("%r" % 1.0)
 print("%d" % 1.0)
 print("%i" % 1.0)
 print("%u" % 1.0)
-print("%x" % 18.0)
-print("%o" % 18.0)
-print("%X" % 18.0)
 
 print("%e" % 1.23456)
 print("%E" % 1.23456)