tests: Verify branch coverage

l0b0 · l0b0 · commit 69f69921ce2e · 2021-07-01T09:26:13.000+12:00
Stricter than statement coverage, so I had to take the coverage minimum down one percentage point. The coverage library does not yet have support for conditional coverage <nedbat/coveragepy#660>, which is even stricter.
diff --git a/.coveragerc b/.coveragerc
@@ -1,10 +1,11 @@
 [report]
-fail_under = 95
+fail_under = 94
 exclude_lines =
     if TYPE_CHECKING:
 
 
 [run]
+branch = true
 source =
     pystac
     tests
