Admin Permissions

Giving or revoking admin permissions can be done with the EditAdminRequest:

from telethon.tl.functions.channels import EditAdminRequest
from telethon.tl.types import ChannelAdminRights

# You need both the channel and who to grant permissions
# They can either be channel/user or input channel/input user.
#
# ChannelAdminRights is a list of granted permissions.
# Set to True those you want to give.
rights = ChannelAdminRights(
    post_messages=None,
    add_admins=None,
    invite_users=None,
    change_info=True,
    ban_users=None,
    delete_messages=True,
    pin_messages=True,
    invite_link=None,
    edit_messages=None
)

client(EditAdminRequest(channel, who, rights))

Thanks to @Kyle2142 for pointing out that you cannot set to True the post_messages and edit_messages fields. Those that are None can be omitted (left here so you know which are available).