Usage of opengl_cb is deprecated. Use https://gist.github.com/jaseg/657e8ecca3267c0d82ec85d40f423caa instead.

mpv_gtk_glarea_widget.py

#!/usr/bin/env python3
import ctypes

import gi

gi.require_version('Gtk', '3.0')
from gi.repository import Gtk, Gdk

gi.require_version('GL', '1.0')
from OpenGL import GL, GLX

from mpv import MPV, _mpv_get_sub_api, _mpv_opengl_cb_set_update_callback, \
    _mpv_opengl_cb_init_gl, OpenGlCbGetProcAddrFn, _mpv_opengl_cb_draw, \
    MpvSubApi, OpenGlCbUpdateFn, _mpv_opengl_cb_uninit_gl


def get_process_address(_, name):
    address = GLX.glXGetProcAddress(name.decode("utf-8"))
    return ctypes.cast(address, ctypes.c_void_p).value


class MainClass(Gtk.Window):

    def __init__(self, media):
        super(MainClass, self).__init__()
        self.media = media
        self.set_default_size(600, 400)
        self.connect("destroy", self.on_destroy)

        frame = Gtk.Frame()
        self.area = OpenGlArea()
        self.area.connect("realize", self.play)
        frame.add(self.area)
        self.add(frame)
        self.show_all()

    def on_destroy(self, widget, data=None):
        Gtk.main_quit()

    def play(self, arg1):
        self.area.play(self.media)


# noinspection PyTypeChecker
class OpenGlArea(Gtk.GLArea):

    def __init__(self, **properties):
        super().__init__(**properties)

        self.mpv = MPV(vo='opengl-cb')
        self.mpv_gl = _mpv_get_sub_api(self.mpv.handle, MpvSubApi.MPV_SUB_API_OPENGL_CB)

        self.on_update_c = OpenGlCbUpdateFn(self.wrapped_c_render_func)
        self.get_proc_addr_c = OpenGlCbGetProcAddrFn(get_process_address)
        _mpv_opengl_cb_set_update_callback(self.mpv_gl, self.on_update_c, None)

        self.connect("realize", self.on_realize)
        self.connect("render", self.on_render)
        self.connect("unrealize", self.on_unrealize)

    def on_realize(self, area):
        self.make_current()
        _mpv_opengl_cb_init_gl(self.mpv_gl, None, self.get_proc_addr_c, None)

    def on_unrealize(self, arg):
        _mpv_opengl_cb_uninit_gl(self.mpv_gl)
        self.mpv.terminate()

    def wrapped_c_render_func(self, ctx=None):
        self.queue_render()

    def on_render(self, arg1, arg2):
        factor = self.get_scale_factor()
        rect = self.get_allocated_size()[0]

        width = rect.width * factor
        height = -rect.height * factor

        _mpv_opengl_cb_draw(self.mpv_gl, GL.glGetIntegerv(GL.GL_DRAW_FRAMEBUFFER_BINDING), width, height)

    def play(self, media):
        self.mpv.play(media)


if __name__ == '__main__':
    import locale

    locale.setlocale(locale.LC_NUMERIC, 'C')

    application = MainClass(media='test.webm')
    Gtk.main()

As of release v.0.29.0 the usage of 'opengl-cb' is deprecated.
Use https://gist.github.com/jaseg/657e8ecca3267c0d82ec85d40f423caa instead.

https://github.com/mpv-player/mpv/releases/tag/v0.29.0
https://github.com/mpv-player/mpv/blob/5feee11e43a0907306db1df993cf1cd11c8596d9/libmpv/opengl_cb.h#L22