devel/py-inflection: Update to 0.5.1

• The support for Python < 3.5 was dropped with the 0.4.0 release, adapt it accordingly.

• Add "do-test" target to make future QA easier.

Changelogs since 0.3.1:

https://github.com/jpvanhal/inflection/releases/tag/0.5.1
https://github.com/jpvanhal/inflection/releases/tag/0.5.0
https://github.com/jpvanhal/inflection/releases/tag/0.4.0