Use index instead of TryConvertToInt64 in socket.__init__

[slozier]

In the 3.6 branch, socket.__init__ uses Converter.TryConvertToInt64 on the fileno. This is incorrect and probably should be something like TryToIndex.

See comment by @BCSharp https://github.com/IronLanguages/ironpython3/pull/1507#issue-1306616504 for details.