* gh-153056: Fix a data race compiling the string.Template pattern in free-threading builds
Template compiles its substitution pattern lazily and caches it on the class. On the free-threaded build two concurrent first uses could race: a thread that observed the pattern another thread had just compiled would try to recompile it, and re.compile() rejects flags on an already-compiled pattern, raising a spurious ValueError. Return the already-compiled pattern instead.
As a side effect, a subclass that supplies an already-compiled pattern now works too; previously it raised the same ValueError at class definition.
* Trim test comments and NEWS wording
* Document that the pattern attribute accepts a string or a compiled regex
* Comment the three states of pattern and note the documented-behavior fix in NEWS
* Update Doc/library/string.rst
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Co-authored-by: Barry Warsaw <barry@python.org>
