Improve Coverage.py configuration (#689)

Hide data files within a directory to avoid cluttering the repository root, and minimize reports.

Author: adamchainz

pyproject.toml

@@ -130,6 +130,7 @@ ini_options.xfail_strict = true
 
 [tool.coverage]
 run.branch = true
+run.data_file = ".coverage/cov"
 run.parallel = true
 run.source = [
   "tests",
@@ -140,6 +141,8 @@ paths.source = [
   ".tox/**/site-packages",
 ]
 report.show_missing = true
+report.skip_covered = true
+report.skip_empty = true
 
 [tool.rstcheck]
 report_level = "ERROR"