python-semanticversion currently appears to support "canonical" NPM version specs, as according to their range.bnf, but the node-semver implementation also accepts a superset of that syntax as valid.
For example, the implementation allows spaces between the prefix and the version (> 1.2.3), or additional whitespace within a hyphen-range (0.1.2 - 3.4.5).
It also has a ~> prefix, which appears to be equivalent to ~.

The NpmSpec does not accept these as valid, and raises a ValueError.

I believe that NpmSpec should aim to be equivalent to the node-semver implementation, correctly parsing anything NPM accepts as valid.

Note also the related issue npm/node-semver#392, which goes into more detail about the discrepancy between the syntax definition and the implementation.