make vstring/version variables in LooseVersion read-only

[Flamefire]

The exposed "public" members .version and .vstring must not be modified from outside as they belong together. Hence put them into properties.

I was also thinking about the case of LooseVersion(None) which can easily happen with LooseVersion(get_software_version('Bazel')). One approach was to compare that equal to None or LooseVersion(None) and "less" otherwise. But that might be surprising and hence better/safer is if LooseVersion(...).version is None

I hence added that to the test suite to show an example and kept my WIP commit in the history if we later decide otherwise.

@boegel This should go into the next release which adds this class.

[boegel]

lgtm

[Flamefire]

There was a use case that caused now test failures: loose_cv.version = loose_cv.version[:depth] in log.deprecated to only consider some parts of the version. I fixed that by comparing the list directly. I guess that works with the caveat that mixed-types in the components are no longer working. But I doubt we'd use that.

I still think this is a good idea because if anyone was to print the loose_cv it would turn up with the original value, not the modified one (which is the reason for this change)

[boegel]

lgtm