Semver 2.0 != 2.0.0 and 2.0.0.0 is valid

[Hritik14]

Code docs say
https://github.com/nexB/univers/blob/a52ea9900d87ef6cd0ed989bf20a41a794cf39c4/src/univers/versions.py#L177-L180

Even so, semver with more than 3 segments is treated as valid. This looks like an issue on the semantic_version side but we should not make false promises.

>>> from univers.versions import SemverVersion
>>> SemverVersion.is_valid("2")
True
>>> SemverVersion.is_valid("2.0")
True
>>> SemverVersion.is_valid("2.0.0")
True
>>> SemverVersion.is_valid("2.0.0.0")
True
>>> SemverVersion.is_valid("2.0.0.0.1")
True
>>> SemverVersion("2") == SemverVersion("2.0")
False
>>> SemverVersion("2.0.0") == SemverVersion("2.0")
False

[pombredanne]

This is actually a bit different:

>>> from univers.versions import SemverVersion
>>> SemverVersion.is_valid("2.0.0.0.1")
True
>>> sv=SemverVersion("2.0.0.0.1")
>>> sv.value
Version('2.0.0+0.1')
>>> v=sv.value
>>> v.major, v.minor, v.patch, v.prerelease, v.build
(2, 0, 0, (), ('0', '1'))

here what happens is that this is semver strict with lenient parsing accepting anything after the patch segment as a build.
We should document this with tests and improved doc alright.
And we could make the parsing strict.