.410 would be an easy choice, but might put a few more holes in your belt.
There are several revolvers on the market now that will chamber .410 shot shell and/or 45 long colt; the Taurus Judge is one. They are kind of bulky for my taste in a CCW roll, but are popular with some ranch owners for their dual roll as varmint gun and in vehicle carry. There are a few companies making dedicated personal protection rounds in .410, too. They usually contain a combination of slugs or larger discs, and pellets.
A quick image search on ".410 shot 45 long colt" found some double barrel side by side pistols that looked like old dragoons. Kinda kool, but maybe not the best tool.
Make sure you are okay to discharge at your location. Most municipalities have restrictions as to how close you can be to other structures (and maybe properties); I think it is typically 150 yds and not within townships, but don't quote me on that.
I don't have any .22 mag or .410 options, so If it were me, and I was legal to discharge on my property (which I am not... except for self defense), I would probably go with my
Ruger Mark III Hunter (.22 LR) similar to this one.
The Cocobolo grips are especially "tasty" when contrasting the stainless and they are known for being "dead eye" accurate. I also have a
Bushnell TRS-25 Reflex Sight for it, but it shoots plenty straight without it and the same gun is available in less expensive variants.
My next choice would be my Marlin 39a (shown previously; takes .22 shorts, longs and long rifle). It's more accurate than the pistol and the peep sight with brass bead is very quick to acquire.
You really don't need anything too powerful, in fact you probably want to keep things pretty subdued around the neighbors. Don't discount an air rifle. They are legal most everywhere, quiet, and the better ones are quite lethal for small game and undesirable pests, such as your snake (as if "lethal" needs an adverb

).
Um, hey, aren't you a pretty handy archer? Get out your favorite bow! Even a wrist rocket can take out a snake with a good head shot, just don't get too close.
If he comes around again, be sure to get a picture!