[issue33791] Update README.rst to mention third-party OpenSSL needed for macOS builds

[Ned Deily]

Ned Deily <nad at python.org> added the comment:

Due to some bot issues, the 3.7 commit message went AWOL: the changeset id is ee994d7443a7e2809a5d49bd3679fc9c451a411b.  Thanks for the PR!

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resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed
versions:  -Python 2.7

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<https://bugs.python.org/issue33791>
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