pypi.bbclass: mind package suffix on version check

Some pypi packages do have suffixes like dev, or a0 or b1.
When doing a version check on these, the version will get falsely
identified as major release versions.
Add a terminating slash to rule out those false positives

Signed-off-by: Konrad Weihmann <kweihmann@outlook.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Steve Sakoman <steve@sakoman.com>