As of this writing (Sept. 2022) OP25 builds for python3 and GNU Radio 3.8.

The full list of supported versions is as follows:

PYTHON 2 AND GNU RADIO 3.7
==========================
It should still be possible to use the file gr3.8.patch (in reverse) to 
downgrade the source tree to build against Python 2 and GNU Radio 3.7,
although this has not been tested.
	$ cat gr3.8.patch | patch -p1 -R
Once this has been done, proceed by running the install.sh script.

PYTHON 3 AND GNU RADIO 3.8
==========================
It is no longer necessary to apply the gr3.8 patch to the op25 source tree,
as Python3/GNU Radio 3.8 is now the default.  You can proceed directly to
running the install.sh script.

PYTHON 3 AND GNU RADIO 3.9 / 3.10
=================================
It is no longer necessary to apply the gr3.8 patch to the op25 source tree,
See the file README-gr3.9 for procedures for both GNU Radio 3.9 and 3.10.