local dev: fix parallel race condition in tox

Author: hynek

tox.ini

@@ -6,7 +6,7 @@ env_list =
     py3{8,9,10,11,12,13,14}-{tests,mypy}
     py3{8,13}-tests-{colorama,be,rich},
     docs-{sponsors,doctests},
-    coverage-report
+    coverage-{combine,report}
 
 
 [testenv]
@@ -32,15 +32,25 @@ deps =
 commands = coverage run -m pytest {posargs}
 
 
-[testenv:coverage-report]
-deps = coverage[toml]
+# Split combine/report in 2 to avoid excessive "Combined data file ..." output.
+[testenv:coverage-combine]
+# Keep base_python in-sync with .python-version-default
+base_python = py313
+# Keep in-sync with test env definition above.
+depends = py3{8,13}-tests{,-colorama,-be,-rich}
 skip_install = true
+deps = coverage
+commands = coverage combine
+
+[testenv:coverage-report]
+# Keep base_python in-sync with .python-version-default
+base_python = py313
+depends = coverage-combine
 parallel_show_output = true
-# Keep in-sync with test env definition above.
-depends = py3{8,13}-tests{,colorama,be,rich}
-commands =
-    coverage combine
-    coverage report
+skip_install = true
+deps = coverage
+commands = coverage report
+
 
 
 [testenv:docs-{build,doctests,linkcheck}]