textproc/py-m2r: Incompletely specified dependency

Hi Sunpoet,

The following commit updated textproc/py-m2r changing its dependency to 
a new port origin, due to mistune API incompatibilities > 2.x but the no 
dependency <version-spec> is declared (>=0)

https://cgit.freebsd.org/ports/commit/?id=995255a5684df6ff950a347555632c569b31365b

> 
> ${PYTHON_PKGNAMEPREFIX}mistune0>=0:textproc/py-mistune0@${PY_FLAVOR}
> 

This causes the following failure at build time for users with mistune 
installed, as the dependency is (incorrectly) satisfied by the installed 
mistune (2.x) version:

>   return '\ :math:`{}`\ '.format(math)
> Traceback (most recent call last):
>   File "<string>", line 1, in <module>
>   File "setup.py", line 14, in <module>
>     from m2r import parse_from_file
>   File "/var/tmp/tmpfs0/usr/home/koobs/repos/freebsd/ports/textproc/py-m2r/work-py39/m2r-0.2.1/m2r.py", line 59, in <module>
>     class RestBlockGrammar(mistune.BlockGrammar):
> AttributeError: module 'mistune' has no attribute 'BlockGrammar'
> *** Error code 1

Please set a correct <version-spec> for the mistune dependency:


   ${PYTHON_PKGNAMEPREFIX}mistune0<2:textproc/py-mistune0@${PY_FLAVOR}

This will cause mistune0 to be installed. The issue of conflicting ports 
is left as a separate exercise.

See also:

https://github.com/miyakogi/m2r/issues/66
https://github.com/miyakogi/m2r/pull/43/files

Thanks,

koobs