Fix compatible prerelease clause (~=7.0.0rc1)

[ddelange]

Before this change the prerelease rc1 in e.g. ~=7.0.0rc1 would be lost on the low end:

>>> vrange = parse_single_constraint('~=7.0.0rc1')
>>> vrange.min
<Version 7.0.0>

After:

>>> vrange = parse_single_constraint('~=7.0.0rc1')
>>> vrange.min
<Version 7.0.0rc1>