meta-python: Convert confusing appends to override syntax

While in this case `RDEPENDS:class-target +=` wouldn't result in any
unwanted override, there is no guarantee there won't be a change, which
would be hidden by this override. To avoid any surprises in the future
let's use `:append:class-target =` syntax here.

Signed-off-by: Michal Sieron <michalwsieron@gmail.com>
Signed-off-by: Khem Raj <khem.raj@oss.qualcomm.com>

meta-python/recipes-devtools/python/python3-msgpack_1.1.2.bb

@@ -8,7 +8,7 @@ PTEST_PYTEST_DIR = "test"
 
 SRC_URI[sha256sum] = "3b60763c1373dd60f398488069bcdc703cd08a711477b5d480eecc9f9626f47e"
 
-RDEPENDS:${PN}:class-target += "\
+RDEPENDS:${PN}:append:class-target = " \
     python3-io \
 "