Add a bump implementation for GentooVersion

[ziadhany]

bump is required to handle revision version ranges correctly for GentooImporterPipeline

see:
aboutcode-org/vulnerablecode#2090 (comment)

Expected:

>>> from univers.versions import GentooVersion
>>> GentooVersion("2.23.3").bump()
GentooVersion(string='2.24')

Actual:

>>> GentooVersion("2.23.3").bump()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'GentooVersion' object has no attribute 'bump'