tox: Don't mention isort in dependencies

isort is already a dependency of pylint, there is no need to mention
it explicitly. Except for the "formatting" environment where we want
to pin a specific version to avoid noise when a new version of
isort is released that reports errors.

tox.ini

@@ -5,7 +5,6 @@ skip_missing_interpreters = true
 [testenv:pylint]
 deps =
    git+https://github.com/pycqa/astroid@master
-   isort
    pytest
 commands =
     # This would be greatly simplified by a solution for https://github.com/PyCQA/pylint/issues/352
@@ -56,7 +55,6 @@ commands =
 deps =
    https://github.com/PyCQA/astroid/tarball/master#egg=astroid-master-2.0
    coverage<5.0
-   isort
    mccabe
    # isort 5 is not compatible with Python 3.5
    py35: isort>=4.2.5,<5
@@ -148,7 +146,6 @@ commands =
 deps =
    https://github.com/PyCQA/astroid/tarball/master#egg=astroid-master-2.0
    coverage<5.0
-   isort
    mccabe
    pytest
    pytest-xdist