python3-pip: fix install rule

Fixes: https://github.com/openwrt/packages/issues/8301

This seems to have slipped for some time. No idea if it ever worked.
It could be that this worked at some point.

In any case, the shebang is properly updated now.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>

lang/python/python3/files/python3-package-pip.mk

@@ -12,7 +12,7 @@ $(call Package/python3/Default)
   DEPENDS:=+python3 +python3-setuptools +python-pip-conf
 endef
 
-define Package/python3-pip/install
+define Py3Package/python3-pip/install
 	$(INSTALL_DIR) $(1)/usr/bin $(1)/usr/lib/python$(PYTHON3_VERSION)/site-packages
 	$(CP) $(PKG_BUILD_DIR)/install-pip/bin/pip3* $(1)/usr/bin
 	$(CP) \