py3: fix doctests with gurobi

[dcoudert]

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.

CC: @seblabbe @jhpalmieri @fchapoton

Component: linear programming

Author: David Coudert

Branch/Commit: 5c6d5fb

Reviewer: Sébastien Labbé

Issue created by migration from https://trac.sagemath.org/ticket/28756

[dcoudert]

Branch: public/linear_programming/28756_gurobi

[dcoudert]

Commit: 5c6d5fb

[dcoudert]

New commits:

5c6d5fb

trac #28756: fix failing doctests and compilation warnings

[dcoudert]

comment:2

if you have gurobi, your help reviewing this ticket will be appreciated ;)

[seblabbe]

comment:3

When Gurobi is the default your branch also fixes many other failures:

sage -t --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

Let me update the description of the ticket.

[seblabbe]

Reviewer: Sébastien Labbé

[dcoudert]

comment:6

Cool, more problems fixed than expected ;)
Thank you !

[vbraun]

Changed branch from public/linear_programming/28756_gurobi to 5c6d5fb