Fix scoped enum class in Cython with a workaround

cython/cython#1603

Author: isuruf

symengine/lib/symengine.pxd

@@ -958,10 +958,11 @@ cdef extern from "<utility>" namespace "std":
     cdef PiecewiseVec std_move_PiecewiseVec "std::move" (PiecewiseVec) nogil
 
 cdef extern from "<symengine/eval.h>" namespace "SymEngine":
-    enum EvalfDomain:
-        EvalfComplex "SymEngine::EvalfDomain::Complex" = 0,
-        EvalfReal "SymEngine::EvalfDomain::Real" = 1,
-        EvalfSymbolic "SymEngine::EvalfDomain::Symbolic" = 2
+    cdef cppclass EvalfDomain:
+        pass
+    cdef EvalfDomain EvalfComplex "SymEngine::EvalfDomain::Complex"
+    cdef EvalfDomain EvalfReal "SymEngine::EvalfDomain::Real"
+    cdef EvalfDomain EvalfSymbolic "SymEngine::EvalfDomain::Symbolic"
     rcp_const_basic evalf(const Basic &b, unsigned long bits, EvalfDomain domain) nogil except +
 
 cdef extern from "<symengine/eval_double.h>" namespace "SymEngine":