Connection initialization is broken on Mac OS

[codeforkjeff]

SQLite on Mac OS doesn't have support for loadable extensions compiled in, so enable_load_extension() fails. This is true for Big Sur's python 2.7.16 and 3.8.2. The python docs also mention this issue.

The fix is to not call enable_load_extension() unless there are extensions to load, so that the adapter can at least work on Mac OS with the pythons that ship with it, w/o extensions. Snapshots will not work due to missing the md5() function. To get those working, people running Mac OS will need to compile sqlite/python or get some other python distribution (maybe Anaconda or Intel's python dist?).