报错：
ImportError: /root/.local/share/letsencrypt/lib/python2.7/site-packages/cryptography/hazmat/bindings/_openssl.so: undefined symbol: OPENSSL_sk_num

解决：

1. yum remove openssl-devel
2. cd ~/.local/share/letsencrypt/bin/
3. ./pip uninstall cryptography pyopenssl -y
4. rm -rf ~/.cache/
5. ./pip install cryptography pyopenssl
6. ldd ~/.local/share/letsencrypt/lib/python2.7/site-packages/cryptography/hazmat/bindings/_openssl.so

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