package/python-attrs: depends on python3

python-attrs contains a file with API for python 3.6+.
Although we can make a "tweak" and remove this specific file
for python2, we prefer to make this package available only for
python3, because the day this package (and many others) will drop
support for python2 is probably very close.

In addition, update python-attrs recursive dependencies to
depend on python3 too (python-automat is the only one left).

Fixes:
 - http://autobuild.buildroot.net/results/aa6/aa6889bc254eaa93adb4fc1a71bcee9f2f23cb33/

Signed-off-by: Asaf Kahlon <asafka7@gmail.com>
[yann.morin.1998@free.fr: fix check-package]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

package/python-attrs/Config.in

@@ -1,5 +1,6 @@
 config BR2_PACKAGE_PYTHON_ATTRS
 	bool "python-attrs"
+	depends on BR2_PACKAGE_PYTHON3
 	help
 	  attrs is the Python package that will bring back the joy of
 	  writing classes by relieving you from the drudgery of

package/python-automat/Config.in

@@ -1,5 +1,6 @@
 config BR2_PACKAGE_PYTHON_AUTOMAT
 	bool "python-automat"
+	depends on BR2_PACKAGE_PYTHON3 # python-attrs
 	select BR2_PACKAGE_PYTHON_ATTRS # runtime
 	select BR2_PACKAGE_PYTHON_SIX # runtime
 	help