Crash at first run when SageMath is also installed

[Cédric M. Campos]

When SageMath is installed, Octave Symbolic will crash at first call and then run properly.

Steps to reproduce on a fresh install of Focal Fossa.

1) Install 'octave-symbolic'.

2) Run Octave and create a symbolic object.

---within Octave---
pkg load symbolic
x = sym('x')
Symbolic pkg v2.8.0: Python communication link active, SymPy v1.5.1.
-------------------

3) Exit Octave and install 'sagemath'.

4) Repeat step 2).

---within Octave---
pkg load symbolic
x = sym('x')
Symbolic pkg v2.8.0: error: SymPy version Setting permissions of DOT_SAGE directory so only you can read and write it.
1.5.1 found but is too old (1.2 required)
    Try "sympref diagnose" for more information.
error: called from
    assert_have_python_and_sympy at line 156 column 9
    python_ipc_popen2 at line 79 column 5
    python_ipc_driver at line 59 column 15
    python_cmd at line 163 column 11
    valid_sym_assumptions at line 38 column 10
    assumptions at line 82 column 7
    check_assumptions>ca_helper at line 41 column 15
    check_assumptions at line 31 column 3
    sym at line 353 column 7
-------------------

5) In the same session, create a symbolic object anew.

---within Octave---
pkg load symbolic
x = sym('x')
Symbolic pkg v2.8.0: Python communication link active, SymPy v1.5.1.
-------------------

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: octave-symbolic 2.8.0-2
ProcVersionSignature: Ubuntu 5.4.0-26.30-generic 5.4.30
Uname: Linux 5.4.0-26-generic x86_64
ApportVersion: 2.20.11-0ubuntu27
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: ubuntu:GNOME
Date: Sun Apr 26 01:43:43 2020
InstallationDate: Installed on 2020-04-24 (1 days ago)
InstallationMedia: Ubuntu 20.04 LTS "Focal Fossa" - Release amd64 (20200423)
PackageArchitecture: all
ProcEnviron:
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=es_ES.UTF-8
 SHELL=/bin/bash
SourcePackage: octave-symbolic
UpgradeStatus: No upgrade log present (probably fresh install)