Trac #28689: py3: fix doctests with cbc

The following doctests are failing with py3 and cbc, but not with py2.
This is due to a different behavior of `round` called in
`get_variable_value`.
{{{
Using --optional=benzene,bliss,build,cbc,csdp,dochtml,dot2tex,gap_packag
es,gfortran,igraph,libsemigroups,mcqd,mpir,plantri,python_igraph,sage,td
lib
Doctesting 1 file using 4 threads.
sage -t --long src/sage/numerical/backends/coin_backend.pyx
**********************************************************************
File "src/sage/numerical/backends/coin_backend.pyx", line 423, in
sage.numerical.backends.coin_backend.CoinBackend.remove_constraint
Failed example:
    p.get_values([x,y])                          # optional - cbc
Expected:
    [0.0, 3.0]
Got:
    [0, 3]
**********************************************************************
File "src/sage/numerical/backends/coin_backend.pyx", line 461, in
sage.numerical.backends.coin_backend.CoinBackend.remove_constraints
Failed example:
    p.get_values(x)                              # optional - cbc
Expected:
    2.0...
Got:
    2
**********************************************************************
File "src/sage/numerical/backends/coin_backend.pyx", line 463, in
sage.numerical.backends.coin_backend.CoinBackend.remove_constraints
Failed example:
    p.get_values(y)                              # optional - cbc
Expected:
    0.0...
Got:
    0
**********************************************************************
File "src/sage/numerical/backends/coin_backend.pyx", line 468, in
sage.numerical.backends.coin_backend.CoinBackend.remove_constraints
Failed example:
    p.get_values([x,y])                          # optional - cbc
Expected:
    [0.0, 3.0]
Got:
    [0, 3]
**********************************************************************
2 items had failures:
   1 of  13 in
sage.numerical.backends.coin_backend.CoinBackend.remove_constraint
   3 of  15 in
sage.numerical.backends.coin_backend.CoinBackend.remove_constraints
    [353 tests, 4 failures, 0.34 s]
}}}

URL: https://trac.sagemath.org/28689
Reported by: dcoudert
Ticket author(s): David Coudert
Reviewer(s): John Palmieri

build/pkgs/cbc/spkg-install

@@ -15,4 +15,4 @@ sed -i -e "s/clock\_gettime ()/Grrrrrrrrrrrr\ ()/g" Cbc/configure || \
 sdh_configure --enable-cbc-parallel --enable-parallel \
               --enable-gnu-packages --enable-static
 sdh_make
-sdh_make_install
+sdh_make_install -j1

src/sage/numerical/backends/coin_backend.pyx

@@ -421,7 +421,7 @@ cdef class CoinBackend(GenericBackend):
             sage: p.solve()                                    # optional - cbc
             10.0
             sage: p.get_values([x,y])                          # optional - cbc
-            [0.0, 3.0]
+            [0, 3]
 
         TESTS:
 
@@ -459,14 +459,14 @@ cdef class CoinBackend(GenericBackend):
             sage: p.solve()                                    # optional - cbc
             9.0
             sage: p.get_values(x)                              # optional - cbc
-            2.0...
+            2
             sage: p.get_values(y)                              # optional - cbc
-            0.0...
+            0
             sage: p.remove_constraints([0])                    # optional - cbc
             sage: p.solve()                                    # optional - cbc
             10.0
             sage: p.get_values([x,y])                          # optional - cbc
-            [0.0, 3.0]
+            [0, 3]
 
         TESTS:
 
@@ -825,7 +825,7 @@ cdef class CoinBackend(GenericBackend):
             sage: p.get_variable_value(1)                          # optional - cbc
             1.5
         """
-        return self.model.solver().getObjValue() + self.obj_constant_term
+        return self.model.solver().getObjValue() + <double>self.obj_constant_term
 
     cpdef get_variable_value(self, int variable):
         r"""
@@ -868,7 +868,7 @@ cdef class CoinBackend(GenericBackend):
         if self.is_variable_continuous(variable):
             return v
         else:
-            return round(v)
+            return int(round(v))
 
     cpdef int ncols(self):
         r"""