Reasonable engines to test against (Not StockFish) I've built a chess engine in Python which I've been testing against StockFish through the use of the Python-Chess package.  

Testing against StockFish seems/is problematic, and as a result I'm looking for suggestions of engines to test against.

There are two major issues with testing against StockFish (the first much more significant than the next)


StockFish is partially used in generating the training data for the engine's evaluation function.  
StockFish is extremely good, and even given extremely little time will perform well.  This makes it difficult to see the changes in the engine's strategy which resulted from a change in the engine's code


The engines I'm hoping to play against need to have no serious bugs or problems, and ideally would play at levels similar or a bit below a human master, without spending too much time running them.  UCI engines are prefered so that my engine can easily communicate with them through the Python-Chess API (note that this doesn't mean my engine supports UCI).

If it helps anyone to have some context, my engine is Batch First.
Since your engine supports UCI, I would suggest checking out lichess.org. You can make a BOT account there and test against several other BOTs and people as well! Also, you can get your BOT rated in different time formats!!

I must warn you that most of the BOTs on lichess some version of StockFish or other strong engine but you can find ample other BOTs.

Among strong BOTs (apart from Stockfish ones), I would suggest checking out:


LeelaChess: A neural net based BOT.
ASMFish


There are many more BOTs on lichess, you can view them here on lichess BOT team.

You can also find many not so strong self-implemented engines that you might like testing against, for example this.