sip: upgrade sip-native and python-sip to 4.7.3

* 4.7.1 had become unfetchable
* verified successful compilation in Sonkei

diff --git a/packages/python/python-sip_4.7.1.bb b/packages/python/python-sip_4.7.3.bb
similarity index 100%
rename from packages/python/python-sip_4.7.1.bb
rename to packages/python/python-sip_4.7.3.bb

diff --git a/packages/sip/sip-native_4.7.1.bb b/packages/sip/sip-native_4.7.3.bb
similarity index 100%
rename from packages/sip/sip-native_4.7.1.bb
rename to packages/sip/sip-native_4.7.3.bb