PendingDeprecationWarning because of spec class in semantic_version

[mulkieran]

We would prefer not to emit it, but we seem to be in some circumstances.

stratis-cli/src/stratis_cli/_actions/_stratisd_version.py:35: PendingDeprecationWarning: The Spec() class will be removed in 3.1; use SimpleSpec() instead.

The problem is that we must run with a somewhat wide range of the semantic_version Python package: 2.6.0 - 2.8.4 inclusive. SimpleSpec was not introduced until 2.7, but the PendingDeprecationWarning was added sometime before 2.8.4.

[mulkieran]

Likely this is due to a new version of unittest or Python that the PendingDeprecationWarning is visible. This is a small consideration for our upgrade to Fedora 32. defaults.

[mulkieran]

Fedora 32 supports 2.8.4, so we are not constrained there. But we should continue to support 2.6.0 for another six months or so.

[mulkieran]

Since only stratis-cli uses Spec, moving there.