gh-153056: Fix a data race compiling the string.Template pattern in free-threading...

gh-153056: Fix a data race compiling the string.Template pattern in free-threading builds (#153057)

* 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>