CI: pin tox at the project level

No sooner do we fix the gate than tox has a new release that breaks
it again. Let's give them a bit to settle down; in the mean time, stick
with 3.x.

See https://github.com/tox-dev/tox/issues/2811

Also simplify our warning suppressions. The message filter is a regex,
so any prefix of the message will suffice. This allows us to also drop a
new message seen on CentOS 8:

   CryptographyDeprecationWarning: Python 3.6 is no longer
   supported by the Python core team. Therefore, support for
   it is deprecated in cryptography. The next release of
   cryptography (40.0) will be the last to support Python 3.6.

As we've previously seen with cryptography warnings, this can slow down
our probe tests to the point that they time out.

Change-Id: I316170442c67c1b4a5b87f9a1168cc04ca2417b8

.zuul.yaml

@@ -659,6 +659,8 @@
        - swift-tox-func-py38-arm64
 
 - project:
+    vars:
+      ensure_tox_version: '<4'
     templates:
       - publish-openstack-docs-pti
       - periodic-stable-jobs

swift/__init__.py

@@ -79,13 +79,7 @@ if (3, 0) <= sys.version_info[:2] <= (3, 5):
 
 warnings.filterwarnings('ignore', module='cryptography|OpenSSL', message=(
     'Python 2 is no longer supported by the Python core team. '
-    'Support for it is now deprecated in cryptography, '
-    'and will be removed in a future release.'))
-warnings.filterwarnings('ignore', module='cryptography|OpenSSL', message=(
-    'Python 2 is no longer supported by the Python core team. '
-    'Support for it is now deprecated in cryptography, '
-    'and will be removed in the next release.'))
+    'Support for it is now deprecated in cryptography'))
 warnings.filterwarnings('ignore', message=(
     'Python 3.6 is no longer supported by the Python core team. '
-    'Therefore, support for it is deprecated in cryptography '
-    'and will be removed in a future release.'))
+    'Therefore, support for it is deprecated in cryptography'))