[3.14] gh-116738: make mmap module thread-safe (GH-139237)

[kumaraditya303]

(cherry picked from commit 7f155f9)

• Issue: Audit all built-in modules for thread safety #116738

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @kumaraditya303 for commit 2c66c93 🤖

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[vstinner]

(cherry picked from commit 7f155f9)

I suppose that it's a straightforward backport. I'm fine with it.

[kumaraditya303]

I suppose that it's a straightforward backport. I'm fine with it.

There's an additional fix 8948ed8 because bd1ada6 wasn't backported but the rest is same.