I'm a bounty hunter so frequent a lot of high intensity res sites but so far haven't seen any pythons or anacondas
(apart from local security)
Is there a level or rank you need to get them to spawn or is it random ?
Combat rank may play a role but ultimately it's random and this randomness is under the factor of two main variables : RES intensity and sector security.
The higher the RES intensity, the higher the chance of getting a great instance (which is big ships spawning regularly).
Same with sector security, the higher the security, the higher the chance of getting a great BH spot.
To be clear, when you enter the instance, a dice is rolled to determine the kind of spawn you're gonna get, which are basically of three possible types : only small ships, small ships mixed with big ships, and only big ships. Intensity (frequency of spawn) may be variable but I can't confirm that yet.