SyntaxWarning: "is not" with a literal #2713

[phillxnet]

Modify conditionals to address Py3.8's added warnings.

Fixes #2713

Testing

Post the proposed changes we have:

lbuildvm:/opt/rockstor/src/rockstor # poetry run python -Wd /opt/rockstor/.venv/bin/django-admin test -v 2
/opt/rockstor/.venv/lib/python3.9/site-packages/django/utils/version.py:98: DeprecationWarning: distutils Version classes are deprecated. Use packaging.version instead.
  loose_version = LooseVersion(version)
Creating test database for alias 'default' ('test_storageadmin')...
...
----------------------------------------------------------------------
Ran 278 tests in 27.020s

OK
...

With the remaining warning pertaining to #2696 Re Py3.10 and our Django.

[phillxnet]

Testing follow-up

• Before, during rpmbuild:

+ cd src/rockstor/
+ poetry run django-admin test
/home/buildbot/worker/Poetry-Build-on-Leap15-5/rpmbuild/rockstor-core-5.0.5-2721/src/rockstor/system/luks.py:241: SyntaxWarning: "is not" with a literal. Did you mean "!="?
  if container_dev is not "":
/home/buildbot/worker/Poetry-Build-on-Leap15-5/rpmbuild/rockstor-core-5.0.5-2721/src/rockstor/system/luks.py:351: SyntaxWarning: "is not" with a literal. Did you mean "!="?
  if len(source_dev_fields) is not 2:
/home/buildbot/worker/Poetry-Build-on-Leap15-5/rpmbuild/rockstor-core-5.0.5-2721/src/rockstor/storageadmin/views/email_client.py:204: SyntaxWarning: "is not" with a literal. Did you mean "!="?
  if len(line) > 0 and line[0] is not "#":
/home/buildbot/worker/Poetry-Build-on-Leap15-5/rpmbuild/rockstor-core-5.0.5-2721/src/rockstor/storageadmin/views/rockon_helpers.py:239: SyntaxWarning: "is not" with a literal. Did you mean "!="?
  if new_state is not "pending_update" and task is not None:
/home/buildbot/worker/Poetry-Build-on-Leap15-5/rpmbuild/rockstor-core-5.0.5-2721/src/rockstor/storageadmin/views/rockon_helpers.py:239: SyntaxWarning: "is not" with a literal. Did you mean "!="?
  if new_state is not "pending_update" and task is not None:
/home/buildbot/worker/Poetry-Build-on-Leap15-5/rpmbuild/rockstor-core-5.0.5-2721/src/rockstor/storageadmin/views/rockon_helpers.py:379: SyntaxWarning: "is" with a literal. Did you mean "=="?
  if c.uid is -1:
Creating test database for alias 'default'...
Creating test database for alias 'smart_manager'...
...

• After proposed change:

+ cd src/rockstor/
+ poetry run django-admin test
Creating test database for alias 'default'...
Creating test database for alias 'smart_manager'...
...

So all linked issue warnings are resolved.