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-humanfriendly_10.0.bb

@@ -10,7 +10,7 @@ SRC_URI[sha256sum] = "6b0b831ce8f15f7300721aa49829fc4e83921a9a301cc7f606be6686a2
 
 inherit pypi setuptools3
 
-RDEPENDS:${PN}:class-target += " \
+RDEPENDS:${PN}:append:class-target = " \
     python3-datetime \
     python3-fcntl \
     python3-io \