Error compiling gurobi backend (undeclared name not building: str_to_bytes)

I get a compilation error when I try to install the gurobi backend by running the command "sage -python -m pip install sage-numerical-backends-gurobi". I have gurobi 9.03 and sage 8.1 installed in Ubuntu 18.04.5 LTS.
$ sage -python -m pip install sage-numerical-backends-gurobi
Collecting sage-numerical-backends-gurobi
  Using cached https://files.pythonhosted.org/packages/27/be/2a9b8999c2e79c9af2432155af6abc6a8a5cb4fc558d2178dbb2e888293d/sage_numerical_backends_gurobi-9.0.0.tar.gz
    Complete output from command python setup.py egg_info:
    Using gurobi_include_directories=['/opt/gurobi903/linux64/include'], libraries=['gurobi90'], library_dirs=['/opt/gurobi903/linux64/lib']
    Checking whether HAVE_SAGE_CPYTHON_STRING...
    Checking whether HAVE_ADD_COL_UNTYPED_ARGS...

    Error compiling Cython file:
    ------------------------------------------------------------
    ...
    from sage.numerical.backends.generic_backend cimport GenericBackend

    cdef class TestBackend(GenericBackend):
        cpdef add_col(self, indices, coeffs):
             ^
    ------------------------------------------------------------

    check_add_col_untyped_args.pyx:4:10: Signature not compatible with previous declaration

    Error compiling Cython file:
    ------------------------------------------------------------
    ...
        cpdef set_verbosity(self, int level)
        cpdef add_linear_constraint(self, coefficients, lower_bound, upper_bound, name=*)
        cpdef add_linear_constraint_vector(self, degree, coefficients, lower_bound, upper_bound, name=*)
        cpdef remove_constraint(self, int)
        cpdef remove_constraints(self, constraints)
        cpdef add_col(self, list indices, list coeffs)
                     ^
    ------------------------------------------------------------

    /usr/lib/python2.7/dist-packages/sage/numerical/backends/generic_backend.pxd:25:18: Previous declaration is here
    Using compile_time_env: {'HAVE_ADD_COL_UNTYPED_ARGS': False, 'HAVE_SAGE_CPYTHON_STRING': False}
    warning: sage_numerical_backends_gurobi/gurobi_backend.pyx:104:10: Compatible but non-identical C method 'add_variable' not redeclared in definition part of extension type 'GurobiBackend'.  This may cause incorrect vtables to be generated.
    warning: /usr/lib/python2.7/dist-packages/sage/numerical/backends/generic_backend.pxd:13:26: Previous declaration is here

    Error compiling Cython file:
    ------------------------------------------------------------
    ...
            cdef char * c_name = ""

            if name is None:
                name = b"x_" + bytes(self.ncols())
            else:
                name = str_to_bytes(name)
                                  ^
    ------------------------------------------------------------

    sage_numerical_backends_gurobi/gurobi_backend.pyx:178:31: undeclared name not builtin: str_to_bytes

    Error compiling Cython file:
    ------------------------------------------------------------
    ...
                else:
                    check(self.env,GRBgetstrattr(self.model, "ModelName", <char **> pp_name))
                    if pp_name[0] == NULL:
                        value = ""
                    else:
                        value = char_to_str(pp_name[0])
                                          ^
    ------------------------------------------------------------

    sage_numerical_backends_gurobi/gurobi_backend.pyx:442:39: undeclared name not builtin: char_to_str
    Compiling sage_numerical_backends_gurobi/gurobi_backend.pyx because it changed.
    [1/1] Cythonizing sage_numerical_backends_gurobi/gurobi_backend.pyx
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/tmp/pip-build-tz2iGm/sage-numerical-backends-gurobi/setup.py", line 165, in <module>
        compile_time_env=compile_time_env),
      File "/usr/lib/python2.7/dist-packages/Cython/Build/Dependencies.py", line 1027, in cythonize
        cythonize_one(*args)
      File "/usr/lib/python2.7/dist-packages/Cython/Build/Dependencies.py", line 1149, in cythonize_one
        raise CompileError(None, pyx_file)
    Cython.Compiler.Errors.CompileError: sage_numerical_backends_gurobi/gurobi_backend.pyx

    ----------------------------------------
sagemath / sage-numerical-backends-gurobi

Error compiling gurobi backend (undeclared name not building: str_to_bytes) #4
