package/dstat: invert select of the python interpreter version

Commit 05484f62 ("package/dstat: bump
to 0.7.4 which allows to run on Python3") added:

   select BR2_PACKAGE_PYTHON if !BR2_PACKAGE_PYTHON3

Unfortunately, some other packages do it the other way around, which
causes the following Kconfig circular dependency:

package/python/Config.in:5:error: recursive dependency detected!
package/python/Config.in:5:	symbol BR2_PACKAGE_PYTHON is selected by BR2_PACKAGE_PYTHON3
package/python3/Config.in:5:	symbol BR2_PACKAGE_PYTHON3 is selected by BR2_PACKAGE_PYTHON

Fix that by inverting the python version selection in the dstat
package.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>