fix find boost-python for cv_bridge

[flynneva]

this should fix the cv_bridge build errors reported for windows #349.

obviously we should try to remove boost going forward for this package.

[flynneva]

from this document:

Note that Boost Python components require a Python version suffix (Boost 1.67 and later), e.g. python36 or python27 for the versions built against Python 3.6 and 2.7, respectively. This also applies to additional components using Python including mpi_python and numpy. Earlier Boost releases may use distribution-specific suffixes such as 2, 3 or 2.7. These may also be used as suffixes, but note that they are not portable.

need to fix the version checks in the cmake file to make it work for Dashing it looks like

[flynneva]

for anyone else who ends up here, boost 1_74_0 is the most recent boost release that still uses python 3.8 (compatible with Foxy).

[Ace314159]

I'd love if this could get merged. I've experienced this same issue, and I have a similar fix that works for me.

[Ace314159]

@mjcarroll Could you please merge this as well?

[mjcarroll]

Replaced by #421