windows: annotate pathcch dependency for pythoncore

This dependency was added in Python 3.9 as part of
bpo-39401 / Git commit 6a65eba44bfd82ccc8bed4b5c6dd6637549955d5.

The backport to Python 3.8 introduced run-time sniffing of the
symbol to avoid the hard library dependency.

This commit adds pathcch as a dependency for pythoncore. No
attention is made to the Python version, effectively meaning we
now require Windows 8 since this library is only available on
Windows 8. I'm fine with not claiming support for Windows 7: that
makes our support story simpler.

This should fix issues with missing symbols for e.g.
PathCchCanonicalizeEx when relinking a new library from the PYTHON.json
metadata.

cpython-windows/build.py

@@ -1753,6 +1753,9 @@ def find_additional_dependencies(project: pathlib.Path):
         {"name": "shlwapi", "system": True},
         {"name": "version", "system": True},
         {"name": "ws2_32", "system": True},
+        # pathcch is Windows 8+ only. Python 3.9 dropped support for Windows 7.
+        # So this dependency is technically incorrect on Python 3.8.
+        {"name": "pathcch", "system": True},
         # In addition to the ones explicitly in the project, there are some
         # implicit link libraries not present. We list those as well.
         {"name": "Ole32", "system": True},

docs/running.rst

@@ -76,6 +76,11 @@ Linux system.
 Windows
 -------
 
+Windows distributions require Windows 8 or Windows Server 2012 or newer. (The
+official Python distributions support Windows 7 up to Python 3.8 and dropped
+support for Windows 7 with Python 3.9. However, we've decided to drop
+support for Windows 7 from our Windows distributions.)
+
 Windows binaries have a dependency on the Microsoft Visual C++ Redistributable,
 likely from MSVC 2015 (``vcruntime140.dll``). This dependency is not
 provided in the distribution and will need to be provided by downstream