python: Drop ${PYTHON_PN}

python 2 is gone and we don't need the abstraction now, drop the remaining usage
of this variable.

The definition in python3-dir.bbclass is left for now for other layers.

(From OE-Core rev: b566b1e32c7993d1ab7795562f648e52ce186a70)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-devtools/python/python3-rfc3339-validator_0.1.4.bb

@@ -13,9 +13,9 @@ UPSTREAM_CHECK_REGEX = "/rfc3339-validator/(?P<pver>(\d+[\.\-_]*)+)/"
 inherit pypi setuptools3
 
 RDEPENDS:${PN} += "\
-    ${PYTHON_PN}-core \
-    ${PYTHON_PN}-datetime \
-    ${PYTHON_PN}-six \
+    python3-core \
+    python3-datetime \
+    python3-six \
 "
 
 BBCLASSEXTEND = "native nativesdk"