Introduce "all" and "none" specifiers

[immqu]

This PR introduces the possibility to specify all versions, or none using the notation vers:all/* and vers:none/*. To allow this, it simply adds a version check in the contains method that returns true if all is specified, and false if none is specified.
Addresses the spec improvement suggested in package-url/purl-spec#267

[tschmidtb51]

Do we check somewhere that the strings are just vers:all/* or vers:none/* instead of something invalid as vers:all/*|>17.4 or vers:none/!4.2?

[tschmidtb51]

@immqu Please also check that your commits are signed (see DCO fails).

[immqu]

Do we check somewhere that the strings are just vers:all/* or vers:none/* instead of something invalid as vers:all/*|>17.4 or vers:none/!4.2?

Currently, it is checked whether the * constraint is included, and if so, no other constraint is allowed. So, no adjustment is necessary to prevent something like vers:all/*|>17.4 (see https://github.com/aboutcode-org/univers/blob/main/src/univers/version_range.py#L141). However, it is possible to specify something like vers:all/>1.2.3 (without the *).
I have added a strict string equality check now, though.

[tschmidtb51]

LGTM

[immqu]

See also the corresponding PR for purl