tests: Verify branch coverage

l0b0 · l0b0 · commit 3d6512b64423 · 2021-06-30T16:17:03.000+12:00
Stricter than statement coverage. The coverage library does not yet have support for conditional coverage <nedbat/coveragepy#660>, which is even stricter.
diff --git a/.coveragerc b/.coveragerc
@@ -5,6 +5,7 @@ exclude_lines =
 
 
 [run]
+branch = true
 source =
     pystac
     tests
