build(test): PyContracts doesn't work on 3.11

and I don't think it ever will.

Author: nedbat

coverage/env.py

@@ -117,4 +117,8 @@ class PYBEHAVIOR:
 # Environment COVERAGE_NO_CONTRACTS=1 can turn off contracts while debugging
 # tests to remove noise from stack traces.
 # $set_env.py: COVERAGE_NO_CONTRACTS - Disable PyContracts to simplify stack traces.
-USE_CONTRACTS = TESTING and not bool(int(os.environ.get("COVERAGE_NO_CONTRACTS", 0)))
+USE_CONTRACTS = (
+    TESTING
+    and not bool(int(os.environ.get("COVERAGE_NO_CONTRACTS", 0)))
+    and (PYVERSION < (3, 11))
+)

tox.ini

@@ -30,8 +30,6 @@ setenv =
     # For some tests, we need .pyc files written in the current directory,
     # so override any local setting.
     PYTHONPYCACHEPREFIX=
-    # PyContracts can't do 3.11.
-    py311: COVERAGE_NO_CONTRACTS=1
 
 commands =
     # Create tests/zipmods.zip