Correctly process source packages in t-p-u

Signed-off-by: Adam D. Barratt <adam@adam-barratt.org.uk>

t-p-u packages are identified by the package name ending in "_tpu", not
by the first character of the name doing so

britney.py

@@ -1981,7 +1981,7 @@ class Britney:
             pkg_name = pkg[1:]
             suite = "testing"
         # testing-proposed-updates = "<source>_tpu"
-        elif pkg[0].endswith("_tpu"):
+        elif pkg.endswith("_tpu"):
             pkg_name = pkg[:-4]
             suite = "tpu"
         # normal update of source packages = "<source>"