Dependencies break with pip 20.3

[joao-p-marques]

pip 20.3 introduced a new dependency resolver that disallows having conflicting dependencies in the same environment. https://pip.pypa.io/en/stable/user_guide/?highlight=resolver#changes-to-the-pip-dependency-resolver-in-20-3-2020

As pylint-odoo requires pylint==2.5.3 (for python 3), a dependency conflict is easily obtained when trying to install it along with pylint:

conflict caused by:
        The user requested isort==4.3.21
        pylint 2.6.0 depends on isort<6 and >=4.2.5
        pylint-odoo 3.5.0 depends on isort==4.3.4

and

        The user requested pylint 2.6.0
        pylint-odoo 3.5.0 depends on pylint==2.5.3

This forces the use of an older pylint and isort version, which is not ideal. What is the reason for requesting that specific version? Can the requirement be more flexible (like >= or similar)? My suggestion would be to support the same isort constraint as the one used by the required pylint version.

cc @Tecnativa TT27232

ping @Yajo