Trac #28756: py3: fix doctests with gurobi

With sage 9.0.beta6 and gurobi, we get 17 failing doctests due to str vs
bytes (`TypeError: expected bytes, str found`), char vs str, and the
change of behavior of zip between py2 and py3 (`TypeError: object of
type 'zip' has no len()`). Some problems were fixed in #28206, but it
was apparently not enough.
Try:
{{{
sage -t --long --optional=sage,optional,external
src/sage/numerical/linear_functions.pyx
src/sage/numerical/linear_tensor.py
src/sage/numerical/linear_tensor_constraints.py
src/sage/numerical/linear_tensor_element.pyx  src/sage/numerical/mip.pyx
src/sage/numerical/backends/gurobi_backend.pyx
}}}
which gives
{{{
----------------------------------------------------------------------
sage -t src/sage/numerical/linear_functions.pyx  # 29 doctests failed
sage -t src/sage/numerical/linear_tensor.py  # 20 doctests failed
sage -t src/sage/numerical/linear_tensor_constraints.py  # 28 doctests
failed
sage -t src/sage/numerical/linear_tensor_element.pyx  # 23 doctests
failed
sage -t src/sage/numerical/mip.pyx  # 4 doctests failed
sage -t src/sage/numerical/backends/gurobi_backend.pyx  # 17 doctests
failed
----------------------------------------------------------------------
}}}

We also fix some compilation warnings.

URL: https://trac.sagemath.org/28756
Reported by: dcoudert
Ticket author(s): David Coudert
Reviewer(s): Sébastien Labbé

src/sage/numerical/backends/gurobi_backend.pyx

@@ -168,7 +168,8 @@ cdef class GurobiBackend(GenericBackend):
 
         if name is None:
             name = b"x_" + bytes(self.ncols())
-
+        else:
+            name = str_to_bytes(name)
         c_name = name
 
         if upper_bound is None:
@@ -182,6 +183,7 @@ cdef class GurobiBackend(GenericBackend):
 
         if coefficients is not None:
 
+            coefficients = list(coefficients)
             nonzeros = len(coefficients)
             c_indices = <int *> sig_malloc(nonzeros * sizeof(int))
             c_coeff = <double *> sig_malloc(nonzeros * sizeof(double))
@@ -518,7 +520,7 @@ cdef class GurobiBackend(GenericBackend):
         """
 
         if lower_bound is None and upper_bound is None:
-            raise ValueError("At least one of 'upper_bound' or 'lower_bound' must be set.")
+            raise ValueError("at least one of 'upper_bound' or 'lower_bound' must be set")
 
         coefficients = list(coefficients)
         cdef int n = len(coefficients)
@@ -549,6 +551,10 @@ cdef class GurobiBackend(GenericBackend):
             else:
                 error = GRBaddrangeconstr(self.model, n, row_i, row_values, <double> lower_bound, <double> upper_bound, str_to_bytes(name))
 
+        else:
+            # This case is repeated here to avoid compilation warnings
+            raise ValueError("at least one of 'upper_bound' or 'lower_bound' must be set")
+
         check(self.env,error)
 
         error = GRBupdatemodel(self.model)
@@ -850,7 +856,7 @@ cdef class GurobiBackend(GenericBackend):
         if name[0] == NULL:
             value = ""
         else:
-            value = str(name[0])
+            value = char_to_str(name[0])
         return value
 
     cpdef row_name(self, int index):
@@ -874,7 +880,7 @@ cdef class GurobiBackend(GenericBackend):
         if name[0] == NULL:
             value = ""
         else:
-            value = str(name[0])
+            value = char_to_str(name[0])
         return value
 
     cpdef bint is_variable_binary(self, int index):
@@ -1152,6 +1158,8 @@ cdef class GurobiBackend(GenericBackend):
             raise ValueError("This parameter is not available. "+
                              "Enabling it may not be so hard, though.")
 
+        name = str_to_bytes(name)
+
         if t == "int":
             if value is None:
                 check(self.env, GRBgetintparam(self.env, name, tmp_int))
@@ -1167,9 +1175,9 @@ cdef class GurobiBackend(GenericBackend):
         elif t == "string":
             if value is None:
                 check(self.env, GRBgetstrparam(self.env, name, c_name))
-                return str(c_name)
+                return char_to_str(c_name)
             else:
-                check(self.env, GRBsetstrparam(self.env, name, value))
+                check(self.env, GRBsetstrparam(self.env, name, str_to_bytes(value)))
         else:
             raise RuntimeError("This should not happen.")