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.

1 files changed, 2 insertions, 0 deletions

diff --git a/PCbuild/pcbuild.sln b/PCbuild/pcbuild.sln
index 6d4c9506e5e..61db4e02ad3 100644
--- a/PCbuild/pcbuild.sln
+++ b/PCbuild/pcbuild.sln
@@ -91,6 +91,8 @@ Project("{8BC9CEB8-8B4A-11D0-8D11-00A0C91BC942}") = "_testconsole", "_testconsol
 EndProject
 Project("{8BC9CEB8-8B4A-11D0-8D11-00A0C91BC942}") = "_asyncio", "_asyncio.vcxproj", "{384C224A-7474-476E-A01B-750EA7DE918C}"
 EndProject
+Project("{8BC9CEB8-8B4A-11D0-8D11-00A0C91BC942}") = "_zoneinfo", "_zoneinfo.vcxproj", "{FCBE1EF2-E0F0-40B1-88B5-00A35D378742}"
+EndProject
 Project("{8BC9CEB8-8B4A-11D0-8D11-00A0C91BC942}") = "_queue", "_queue.vcxproj", "{78D80A15-BD8C-44E2-B49E-1F05B0A0A687}"
 EndProject
 Project("{8BC9CEB8-8B4A-11D0-8D11-00A0C91BC942}") = "liblzma", "liblzma.vcxproj", "{12728250-16EC-4DC6-94D7-E21DD88947F8}"