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-psutil_5.9.8.bb

@@ -16,17 +16,17 @@ FILES:${PN}-tests += " \
 
 
 RDEPENDS:${PN} += " \
-    ${PYTHON_PN}-shell \
-    ${PYTHON_PN}-threading \
-    ${PYTHON_PN}-xml \
-    ${PYTHON_PN}-netclient \
-    ${PYTHON_PN}-ctypes \
-    ${PYTHON_PN}-resource \
+    python3-shell \
+    python3-threading \
+    python3-xml \
+    python3-netclient \
+    python3-ctypes \
+    python3-resource \
 "
 
 RDEPENDS:${PN}-tests += " \
      ${PN} \
-     ${PYTHON_PN} \
+     python3 \
      coreutils \
      procps \
      binutils \