package/mongodb: needs host-python2

package/mongodb: needs host-python2

mongodb (like gnuradio) needs host-python2 however there is no way to
enforce this so add a dependency on !BR2_PACKAGE_PYTHON3.
Indeed, if BR2_PACKAGE_PYTHON3 is selected, then buildroot will only
build and install host-python-typing for host-python3.

This issue was not raised in the previous version of mongodb as
host-scons was the only dependency however we now have
host-python-typing and host-python-pyyaml dependencies and it
does not seem right to enforce python2 on those packages

Fixes:
 - http://autobuild.buildroot.org/results/693bdba2c01a1b69f56d6ee75094a6a0fc3f40b4

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[Thomas: propagate dependency to Config.in comment]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>