bpo-40503: PEP 615: Tests and implementation for zoneinfo (GH-19909)

This is the initial implementation of PEP 615, the zoneinfo module,
ported from the standalone reference implementation (see
https://www.python.org/dev/peps/pep-0615/#reference-implementation for a
link, which has a more detailed commit history).

This includes (hopefully) all functional elements described in the PEP,
but documentation is found in a separate PR. This includes:

1. A pure python implementation of the ZoneInfo class
2. A C accelerated implementation of the ZoneInfo class
3. Tests with 100% branch coverage for the Python code (though C code
   coverage is less than 100%).
4. A compile-time configuration option on Linux (though not on Windows)

Differences from the reference implementation:

- The module is arranged slightly differently: the accelerated module is
  `_zoneinfo` rather than `zoneinfo._czoneinfo`, which also necessitates
  some changes in the test support function. (Suggested by Victor
  Stinner and Steve Dower.)
- The tests are arranged slightly differently and do not include the
  property tests. The tests live at test/test_zoneinfo/test_zoneinfo.py
  rather than test/test_zoneinfo.py or test/test_zoneinfo/__init__.py
  because we may do some refactoring in the future that would likely
  require this separation anyway; we may:
        - include the property tests
        - automatically run all the tests against both pure Python and C,
          rather than manually constructing C and Python test classes (similar
          to the way this works with test_datetime.py, which generates C
          and Python test cases from datetimetester.py).
- This includes a compile-time configuration option on Linux (though not
  on Windows); added with much help from Thomas Wouters.
- Integration into the CPython build system is obviously different from
  building a standalone zoneinfo module wheel.
- This includes configuration to install the tzdata package as part of
  CI, though only on the coverage jobs. Introducing a PyPI dependency as
  part of the CI build was controversial, and this is seen as less of a
  major change, since the coverage jobs already depend on pip and PyPI.

Additional changes that were introduced as part of this PR, most / all of
which were backported to the reference implementation:

- Fixed reference and memory leaks

    With much debugging help from Pablo Galindo

- Added smoke tests ensuring that the C and Python modules are built

    The import machinery can be somewhat fragile, and the "seamlessly falls
    back to pure Python" nature of this module makes it so that a problem
    building the C extension or a failure to import the pure Python version
    might easily go unnoticed.

- Adjustments to zoneinfo.__dir__

    Suggested by Petr Viktorin.

- Slight refactorings as suggested by Steve Dower.

- Removed unnecessary if check on std_abbr

    Discovered this because of a missing line in branch coverage.

Lib/test/test_zoneinfo/_support.py

@@ -0,0 +1,76 @@
+import contextlib
+import functools
+import sys
+import threading
+import unittest
+from test.support import import_fresh_module
+
+OS_ENV_LOCK = threading.Lock()
+TZPATH_LOCK = threading.Lock()
+TZPATH_TEST_LOCK = threading.Lock()
+
+
+def call_once(f):
+    """Decorator that ensures a function is only ever called once."""
+    lock = threading.Lock()
+    cached = functools.lru_cache(None)(f)
+
+    @functools.wraps(f)
+    def inner():
+        with lock:
+            return cached()
+
+    return inner
+
+
+@call_once
+def get_modules():
+    """Retrieve two copies of zoneinfo: pure Python and C accelerated.
+
+    Because this function manipulates the import system in a way that might
+    be fragile or do unexpected things if it is run many times, it uses a
+    `call_once` decorator to ensure that this is only ever called exactly
+    one time — in other words, when using this function you will only ever
+    get one copy of each module rather than a fresh import each time.
+    """
+    import zoneinfo as c_module
+
+    py_module = import_fresh_module("zoneinfo", blocked=["_zoneinfo"])
+
+    return py_module, c_module
+
+
+@contextlib.contextmanager
+def set_zoneinfo_module(module):
+    """Make sure sys.modules["zoneinfo"] refers to `module`.
+
+    This is necessary because `pickle` will refuse to serialize
+    an type calling itself `zoneinfo.ZoneInfo` unless `zoneinfo.ZoneInfo`
+    refers to the same object.
+    """
+
+    NOT_PRESENT = object()
+    old_zoneinfo = sys.modules.get("zoneinfo", NOT_PRESENT)
+    sys.modules["zoneinfo"] = module
+    yield
+    if old_zoneinfo is not NOT_PRESENT:
+        sys.modules["zoneinfo"] = old_zoneinfo
+    else:  # pragma: nocover
+        sys.modules.pop("zoneinfo")
+
+
+class ZoneInfoTestBase(unittest.TestCase):
+    @classmethod
+    def setUpClass(cls):
+        cls.klass = cls.module.ZoneInfo
+        super().setUpClass()
+
+    @contextlib.contextmanager
+    def tzpath_context(self, tzpath, lock=TZPATH_LOCK):
+        with lock:
+            old_path = self.module.TZPATH
+            try:
+                self.module.reset_tzpath(tzpath)
+                yield
+            finally:
+                self.module.reset_tzpath(old_path)