Set PYTHONWARNINGS only for the test run itself.

Other things (the install :/) are just too noisy themselves,
and raise errors.

Author: Julian

tox.ini

@@ -13,7 +13,6 @@ changedir = {envtmpdir}
 passenv = CODECOV* CI
 setenv =
     JSON_SCHEMA_TEST_SUITE = {toxinidir}/json
-    tests: PYTHONWARNINGS = error,ignore:Creating a LegacyVersion:DeprecationWarning,ignore:SelectableGroups dict interface:DeprecationWarning,ignore:":UserWarning,ignore:the imp module:DeprecationWarning,ignore:The distutils:DeprecationWarning
 
     coverage,codecov: MAYBE_COVERAGE = coverage run -m
     coverage,codecov: COVERAGE_RCFILE={toxinidir}/.coveragerc
@@ -28,7 +27,7 @@ commands =
     format,perf: {envpython} -m pip install '{toxinidir}[format]'
     format_nongpl: {envpython} -m pip install '{toxinidir}[format_nongpl]'
 
-    tests,coverage,codecov: {envpython} -m {env:MAYBE_COVERAGE:} twisted.trial {posargs:jsonschema}
+    tests,coverage,codecov: {envpython} -W 'error' -m {env:MAYBE_COVERAGE:} twisted.trial {posargs:jsonschema}
     tests: {envpython} -m doctest {toxinidir}/README.rst
 
     coverage: {envpython} -m coverage report --show-missing