test: Add html coverage reports from pytest (#1771)

* Add pytest tool options for html coverage reports.

Author: matthewfeickert

pyproject.toml

@@ -47,9 +47,10 @@ addopts = [
     "--ignore=binder/",
     "--ignore=docs/",
     "--cov=pyhf",
-    "--cov-report=term-missing",
     "--cov-config=.coveragerc",
+    "--cov-report=term-missing",
     "--cov-report=xml",
+    "--cov-report=html",
     "--doctest-modules",
     "--doctest-glob='*.rst'"
 ]