Pin astroid version to fix pylint recursion error

This fixes a problem we've encountered where pylint tests fail with
this error:

	RecursionError: maximum recursion depth exceeded in comparison

The bug appears to be this one:

	pylint-dev/pylint#3318

For the record, this was extremely hard to track down.  The error
appeared erratically, as there appears to be some caching (and
invalidation + recreation) of tox environments and/or packages that
feeds into this.

We can revisit the version pinning in the future; at the moment,
there's no other motivation for the pinning.

Signed-off-by: Hugh Brown (Saint Aardvark the Carpeted) <aardvark@saintaardvarkthecarpeted.com>

tox.ini

@@ -5,6 +5,7 @@ envlist = flake8,isort,yapf,pylint,build,pytest
 flake8 = 3.7.7
 isort = 4.3.21
 yapf = 0.27.0
+astroid = 2.2.5
 pylint = 2.3.1
 pytest = 4.4.2
 pytest-cov = 2.7.1
@@ -53,6 +54,7 @@ commands = pytest -v --cov=polaris --cov=contrib tests
 deps =
     pylint=={[depversions]pylint}
     pytest=={[depversions]pytest}
+    astroid=={[depversions]astroid}
 commands = pylint \
     tests \
     {envsitepackagesdir}/polaris \