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.

setup.py

@@ -304,6 +304,17 @@ def find_library_file(compiler, libname, std_dirs, paths):
     else:
         assert False, "Internal error: Path not found in std_dirs or paths"
 
+def validate_tzpath():
+    base_tzpath = sysconfig.get_config_var('TZPATH')
+    if not base_tzpath:
+        return
+
+    tzpaths = base_tzpath.split(os.pathsep)
+    bad_paths = [tzpath for tzpath in tzpaths if not os.path.isabs(tzpath)]
+    if bad_paths:
+        raise ValueError('TZPATH must contain only absolute paths, '
+                         + f'found:\n{tzpaths!r}\nwith invalid paths:\n'
+                         + f'{bad_paths!r}')
 
 def find_module_file(module, dirlist):
     """Find a module in a set of possible folders. If it is not found
@@ -816,6 +827,8 @@ class PyBuildExt(build_ext):
         # uses modf().
         self.add(Extension('_datetime', ['_datetimemodule.c'],
                            libraries=['m']))
+        # zoneinfo module
+        self.add(Extension('_zoneinfo', ['_zoneinfo.c'])),
         # random number generator implemented in C
         self.add(Extension("_random", ["_randommodule.c"],
                            extra_compile_args=['-DPy_BUILD_CORE_MODULE']))
@@ -2495,6 +2508,7 @@ def main():
         ProcessPoolExecutor = None
 
     sys.modules['concurrent.futures.process'] = DummyProcess
+    validate_tzpath()
 
     # turn off warnings when deprecated modules are imported
     import warnings