Explicitly install `packaging`

It looks like this used to be installed automatically, but it's not anymore.

tf_sig_build_dockerfiles/devel.requirements.txt

@@ -13,6 +13,7 @@ h5py ~= 3.6.0
 keras_preprocessing ~= 1.1.2
 numpy ~= 1.21.4
 opt_einsum ~= 3.3.0
+packaging ~= 21.3
 protobuf ~= 3.19.3
 six ~= 1.16.0
 termcolor ~= 1.1.0