security/py-pyblake2: deprecate and set to removal

External pyblake2 implementation was imported into python standard library
hashlib module since 3.6

https://github.com/dchest/pyblake2/blob/master/README.rst

The only consumers of it (security/py-pyaff4 and security/py-trezor) have
corresponding PRs to drop this dependency.

PR: 277135
Approved by: antoine (maintainer)