bpo-40275: Avoid importing asyncio in test.support (GH-19600)

* Import asyncio lazily in unittest (only when IsolatedAsyncioTestCase is used).
* Import asyncio.events lazily in test.support.

Lib/unittest/__init__.py

@@ -57,7 +57,6 @@ __all__.extend(['getTestCaseNames', 'makeSuite', 'findTestCases'])
 __unittest = True
 
 from .result import TestResult
-from .async_case import IsolatedAsyncioTestCase
 from .case import (addModuleCleanup, TestCase, FunctionTestCase, SkipTest, skip,
                    skipIf, skipUnless, expectedFailure)
 from .suite import BaseTestSuite, TestSuite
@@ -66,6 +65,7 @@ from .loader import (TestLoader, defaultTestLoader, makeSuite, getTestCaseNames,
 from .main import TestProgram, main
 from .runner import TextTestRunner, TextTestResult
 from .signals import installHandler, registerResult, removeResult, removeHandler
+# IsolatedAsyncioTestCase will be imported lazily.
 
 # deprecated
 _TextTestResult = TextTestResult
@@ -78,3 +78,18 @@ def load_tests(loader, tests, pattern):
     # top level directory cached on loader instance
     this_dir = os.path.dirname(__file__)
     return loader.discover(start_dir=this_dir, pattern=pattern)
+
+
+# Lazy import of IsolatedAsyncioTestCase from .async_case
+# It imports asyncio, which is relatively heavy, but most tests
+# do not need it.
+
+def __dir__():
+    return globals().keys() | {'IsolatedAsyncioTestCase'}
+
+def __getattr__(name):
+    if name == 'IsolatedAsyncioTestCase':
+        global IsolatedAsyncioTestCase
+        from .async_case import IsolatedAsyncioTestCase
+        return IsolatedAsyncioTestCase
+    raise AttributeError(f"module {__name__!r} has no attribute {name!r}")