d/control: nfs-common: Move python3 to Recommends

python3 is used in the mountstats, nfsiostat and rpcctl tool shipped in the nfs-common package. While python3 is required to run these tools, for the nfs-common package as whole it is a strong, but not absolute necessary dependency and we can avoid introducing another binary package just to ship those three tools.

Making python3 a Recommends should help minimal installations which do not need or want to pull in python3.

Closes: #1013868