Cannot convert VersionSpecifier to canonical representation and back

[Hritik14]

The offending code is:
https://github.com/nexB/univers/blob/63bd5aec16ec95b5b811ede638ac225f3ab1f6c6/src/univers/version_specifier.py#L147-L153

Further
self.ranges is a VersionRange object and is converted to str by the following block
https://github.com/nexB/univers/blob/63bd5aec16ec95b5b811ede638ac225f3ab1f6c6/src/univers/version_range.py#L84-L85

the self.version is of type BaseVersion and is converted to str by
https://github.com/nexB/univers/blob/63bd5aec16ec95b5b811ede638ac225f3ab1f6c6/src/univers/versions.py#L51-L52

Which yields a string like
<=semver:1.20.0,>=semver:0.6.18 with multiple schemes which is incompatible to be converted back via
https://github.com/nexB/univers/blob/63bd5aec16ec95b5b811ede638ac225f3ab1f6c6/src/univers/version_specifier.py#L100-L104

Possible [dirty] fix:

        ranges = ",".join([f"{rng.operator}{rng.version.value}" for rng in self.version_specifier.ranges])
        return f"{self.scheme}:{ranges}"

[Hritik14]

The vers specification (#12) fixes this very nicely.

__str__ now looks like
https://github.com/nexB/univers/blob/a52ea9900d87ef6cd0ed989bf20a41a794cf39c4/src/univers/version_range.py#L95-L97