textproc/py3-chardet: Convert to slave port

The last update to www/py-requests [1] requires chardet >= 3.0.2. This commit
resulted in pkg-fallout notifications for the www/py3-requests port indicating
that the dependency version could not be satisfied:

py36-requests-2.17.3 depends on package: py36-chardet>=3.0.2 - not found

The textproc/py-chardet port version is 3.0.2, satisfying the dependency
and its minimum version requirement, but the textproc/py3-chardet port version
is 2.3.0. This is due to the py3-chardet being a copy of, not a slave of, and
inheriting values from py-chardet.

py3-foo ports are currently a workaround, used to provide Python 3.x versions of
dependencies for those ports that are built with Python 3.x.

They must be equivalent (in particular in version) to the original port except
for overriding the version of python that it will be built with, and any
dependencies.

Convert textproc/py3-chardet to a slave port accordingly

[1] https://svnweb.freebsd.org/changeset/ports/442565

Reported by: pkg-fallout
Approved by: koobs (python, with hat)