Handle whitespaces for PEP-440 constraints

[abn]

• fix incorrect parsing of spaces when parsing version constraints
• add tests for parse_constraint

This issue was discovered while installing vendoring package. This package uses ~= 3.8.0 for its python version requirement, which was failing due to the whitespace.

$ poetry add -D vendoring
Using version ^0.2.2 for vendoring

Updating dependencies
Resolving dependencies... (0.0s)

[ValueError]
Could not parse version constraint: ~= 3.8.0

[kasteph]

Looks good -- LGTM 💫