build: try “python3” as a last resort for 3.x

So that Xcode's Python 3 gets picked up.

PR-URL: #35983
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Christian Clauss <cclauss@me.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

configure

@@ -10,6 +10,7 @@ command -v python3.8 >/dev/null && exec python3.8 "$0" "$@"
 command -v python3.7 >/dev/null && exec python3.7 "$0" "$@"
 command -v python3.6 >/dev/null && exec python3.6 "$0" "$@"
 command -v python3.5 >/dev/null && exec python3.5 "$0" "$@"
+command -v python3 >/dev/null && exec python3 "$0" "$@"
 command -v python2.7 >/dev/null && exec python2.7 "$0" "$@"
 exec python "$0" "$@"
 ''' "$0" "$@"