python-twisted: avoid downloading build dependencies

setup.py will attempt to download python-incremental, as it is not
present in the sysroot. This is at the least wrong since some random
version of python-incremental will be used to build against (akin to
host contamination) and at most a build failure (when network access
is unavailable). So make python-incrementa-native available and a
DEPENDS.

Unfortunately python-incremental has a dependency on python-twisted
and vice versa. So we need to forcefully break this circular
dependency. Fortunately this doesn't affect the build of
python-twisted. Unfortunately if anyone else attempts to use
python-incremental-native there might be issues should it ever run the
code that does the import of twisted. This is not an issue now and a
comment has been added to hopefully make this issue known to anyone
else coming along at a later time.

Signed-off-by: Mark Asselstine <mark.asselstine@windriver.com>
Signed-off-by: Armin Kuster <akuster808@gmail.com>

meta-python/recipes-devtools/python/python-twisted.inc

@@ -39,6 +39,10 @@ PACKAGES =+ "\
     ${PN}-bin \
 "
 
+DEPENDS += " \
+    ${PYTHON_PN}-incremental-native \
+"
+
 RDEPENDS_${PN} = "\
     ${PN}-bin \
     ${PN}-core \