Build CPython 3.13 by default (#1950)

* Build CPython 3.13 by default

Enable building with CPython 3.13 by default, no longer requiring prerelease_pythons flag to be set true.

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* README.md: Add note about free-threaded support

* build_selector_test.py: cp313t still behind flag

Even with 3.13 being enabled by default, cp313t is still behind a flag for now.

This commit updates the test accordingly.

* fix unit tests

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: mayeut <mayeut@users.noreply.github.com>

cibuildwheel/util.py

@@ -248,7 +248,7 @@ class BuildSelector:
     requires_python: SpecifierSet | None = None
 
     # a pattern that skips prerelease versions, when include_prereleases is False.
-    PRERELEASE_SKIP: ClassVar[str] = "cp313-* cp313t-*"
+    PRERELEASE_SKIP: ClassVar[str] = ""
     prerelease_pythons: bool = False
 
     free_threaded_support: bool = False