DoubleTable
DoubleTable uses box drawing characters for table borders. On Windows terminaltables uses code page 437 characters. However there is no equivalent character set for POSIX (Linux/OS X). Python automatically converts CP437 double-line box characters to Unicode and displays that instead.

Gaps on Windows 10
Like SingleTable the console on Windows 10 changed the default font face to Consolas
. This new font seems to show
gaps between lines. Switching the font back to Lucida Console
eliminates the gaps.
Gaps on POSIX
There is no easy trick for POSIX like there is on Windows. I can’t seem to find out how to force terminals to eliminate gaps vertically between Unicode characters.