Fix cython syntax, add tests

[bjodah]

See gh-480 and cython/cython#6218

Fixes #480
Fixes #483

[bjodah]

added 7472cce since building with cython master gives:

      Error compiling Cython file:
      ------------------------------------------------------------
      ...
      
          def __int__(self):
              return int(float(self))
      
          def __long__(self):
              return long(float(self))
                     ^
      ------------------------------------------------------------
      
      symengine_wrapper.pyx:1208:15: undeclared name not builtin: long
      gmake[2]: *** [symengine/lib/CMakeFiles/symengine_wrapper.dir/build.make:76: symengine/lib/symengine_wrapper.cpp] Error 1
      gmake[1]: *** [CMakeFiles/Makefile2:112: symengine/lib/CMakeFiles/symengine_wrapper.dir/all] Error 2
      gmake: *** [Makefile:136: all] Error 2
      error: error building project
      [end of output]

[bjodah]

Added dd3dc6e since it fixes gh-483

The flint.types.nmod.nmod return type is causing some problems for me locally. I don't think we have a configuration in our CI which actually tests with https://github.com/flintlib/python-flint installed? I guess we should add one such config too.

[oscarbenjamin]

I think that the premise of this test is incorrect:

symengine.py/symengine/tests/test_sympy_conv.py

Lines 771 to 772 in b87a4a2

	a = sympy.FF(7)(3)
	b = sympify(a)

It should be a TypeError to sympify a domain element.

Many other domain/polys types in SymPy inherit CantSympify to enforce this:

$ git grep CantSympify sympy/polys/
sympy/polys/fields.py:from sympy.core.sympify import CantSympify, sympify
sympy/polys/fields.py:class FracElement(DomainElement, DefaultPrinting, CantSympify):
sympy/polys/polyclasses.py:from sympy.core.sympify import CantSympify
sympy/polys/polyclasses.py:class DMP(CantSympify):
sympy/polys/polyclasses.py:class DMF(PicklableWithSlots, CantSympify):
sympy/polys/polyclasses.py:class ANP(CantSympify):
sympy/polys/rings.py:from sympy.core.sympify import CantSympify, sympify
sympy/polys/rings.py:class PolyElement(DomainElement, DefaultPrinting, CantSympify, dict):

I consider it an oversight that ModularInteger does not inherit CantSympify. It isn't possible to enforce for ZZ and QQ but for every other domain we can make sympification fail.