python: don't wipe RDEPENDS when parsing manifest

We want to allow the main recipe to extend RDEPENDS directly, so don't wipe
RDEPENDS when reading the manifest.

This fixes the missing python-misc dependency from python-modules.

As the wiping was having the good side-effect of removing the PN-dev dependency
on PN (which doesn't exist), clear RDEPENDS_${PN}-dev.

(From OE-Core rev: fe3727af217dce4488f1fc6aab3f66232cf11fea)

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-devtools/python/python3_3.5.6.bb

@@ -245,6 +245,8 @@ FILES_${PN}-man = "${datadir}/man"
 
 RDEPENDS_${PN}-ptest = "${PN}-modules ${PN}-tests unzip bzip2"
 
+RDEPENDS_${PN}-dev = ""
+
 BBCLASSEXTEND = "nativesdk"
 
 RPROVIDES_${PN} += "${PN}-modules"
@@ -288,7 +290,6 @@ python(){
             for value in python_manifest[key]['cached']:
                     d.appendVar('FILES_' + pypackage, ' ' + value)
 
-        d.setVar('RDEPENDS_' + pypackage, '')
         for value in python_manifest[key]['rdepends']:
             # Make it work with or without $PN
             if '${PN}' in value: