原因：版本太低，安装就好了。

ERROR: pyopenssl 19.0.0 has requirement cryptography>=2.3, but you'll have cryptography 2.1.4 which is incompatible.
Installing collected packages: ldap3, hyperframe, click, zstandard, protobuf, ruamel.yaml.clib, ruamel.yaml, hpack, h2, pyperclip, publicsuffix2, urwid, pydivert, blinker, h11, wsproto, pyparsing, Jinja2, Werkzeug, flask, passlib, sortedcontainers, kaitaistruct, certifi, pyOpenSSL, Brotli, mitmproxy
  Found existing installation: click 6.7
    Uninstalling click-6.7:
      Successfully uninstalled click-6.7
  Found existing installation: pyparsing 2.2.0
    Uninstalling pyparsing-2.2.0:
      Successfully uninstalled pyparsing-2.2.0
  Found existing installation: Jinja2 2.10
    Uninstalling Jinja2-2.10:
      Successfully uninstalled Jinja2-2.10
  Found existing installation: Werkzeug 0.14.1
    Uninstalling Werkzeug-0.14.1:
      Successfully uninstalled Werkzeug-0.14.1
  Found existing installation: Flask 1.0.2
    Uninstalling Flask-1.0.2:
      Successfully uninstalled Flask-1.0.2
  Found existing installation: certifi 2018.4.16
    Uninstalling certifi-2018.4.16:
      Successfully uninstalled certifi-2018.4.16
  Found existing installation: pyOpenSSL 17.5.0
    Uninstalling pyOpenSSL-17.5.0:

下载：https://pypi.org/simple/pyopenssl/