package/rust: allow using python3 interpreter

package/rust: allow using python3 interpreter

Fedora packaging use python3 as python interpreter since rust 1.24.0 [1]
by removing python2 tests from configure script [2].

Using python3 will help to remove python2 in a near future.

[1] https://src.fedoraproject.org/rpms/rust/c/216b2d27716bf1031c526dbd0e01a1fa8e6d5aa2
[2] https://src.fedoraproject.org/rpms/rust/blob/216b2d27716bf1031c526dbd0e01a1fa8e6d5aa2/f/rust.spec#_314

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Titouan Christophe <titouan.christophe@railnova.eu>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Patrick Havelange <patrick.havelange@essensium.com>
Tested-by: Titouan Christophe <titouan.christophe@railnova.eu>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>