I vote for enabling SSL_MODE_ACCEPT_MOVING_WRITE_BUFFER by default.  It can catch mistakes (i.e. failure to check the return value of send) in Python just as easily as in C, but I don't think those mistakes are common enough to be worth the headache of this error.  The false positive rate of this error is higher in Python than in C because we don't have direct control over memory and pointers.

As for partial writes, I'm not sure if it's backwards compatible to turn them on by default, but it might be nice if the option were exposed. Partial writes may have less benefit in Python than in C since we'd have to reallocate and copy a string instead of just moving a pointer.