Support Python 3.7 in external probe scanner code

[acammack-r7]

Removes a deprecated async declaration that was removed in 3.7. Minimum Python version to run the probe scanner code is now 3.5.3.

Verification

• With Python 3.7
• PYTHONPATH=$(pwd)/lib/msf/core/modules/external/python ./modules/auxiliary/scanner/wproxy/att_open_proxy.py --rhosts 127.0.0.1
• Verify you get a "Connect failed" error and not a Python TypeError

[busterb]

$ PYTHONPATH=$(pwd)/lib/msf/core/modules/external/python ./modules/auxiliary/scanner/wproxy/att_open_proxy.py --rhosts 127.0.0.1
[!] 127.0.0.1:49152 - Error connecting: [Errno 61] Connect call failed ('127.0.0.1', 49152)

Yep, good fix.

[busterb]

Release Notes

This fix adds Python 3.7 support for external Python scanner modules and raises the minimum Python 3 version to 3.5.3.