user16593: 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.

user11328: Try https://github.com/fernandotenorio/Tunguska
And let me know if your Python engine beat mine! =D

user16910: To complement the list given by @SmallChess, you can find at https://www.computerchess.org.uk/ccrl/4040/ an ELO ranking of many chess engines.
In the 2300-2600 range ("similar or a bit below a human master"), and amongst open-source engines, you have : 

K2 0.87 (2580 ELO)
Jazz 840 64-bit 4CPU (2552 ELO)
Maverick 1.5 64-bit (2522 ELO)
Winter 0.1.1 64-bit (2492 ELO)
Lozza 1.18 64-bit (2452 ELO)
Topple 0.1.1 64-bit (2360 ELO)

I haven't tested these engines myself, but they seem to fit your criterias.
Update : If you're looking more for engines in the 2000-2300 range, then these engines might suit you better : 

Absolute Zero 2.4.7.2 64-bit (ELO 2294)
CT800 1.30 64-bit (ELO 2253)
Zevra 1.8.4 r650 64-bit (ELO 2236)
ZCT 0.3.2451 (ELO 2210)
Tunguska 1.0 64-bit (ELO 2133)
Cinnamon 2.0 64-bit (ELO 2054)


user825: http://computer-chess.org/doku.php?id=computer_chess:wiki:lists:chess_engine_list

Arasan
GreKo
Crafty
Rodent
Toga etc


user11241: 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.

16593: <p>I've built a chess engine in Python which I've been testing against StockFish through the use of the <a href="https://github.com/niklasf/python-chess" rel="nofollow noreferrer">Python-Chess</a> package.  </p>

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

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

<ol>
<li>StockFish is partially used in generating the training data for the engine's evaluation function.  </li>
<li>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</li>
</ol>

<p>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).</p>

<p>If it helps anyone to have some context, my engine is <a href="https://github.com/SamRagusa/Batch-First" rel="nofollow noreferrer">Batch First</a>.</p>
 11328: <p>Try <a href="https://github.com/fernandotenorio/Tunguska" rel="nofollow noreferrer">https://github.com/fernandotenorio/Tunguska</a>
And let me know if your Python engine beat mine! =D</p>
 16910: <p>To complement the list given by @SmallChess, you can find at <a href="https://www.computerchess.org.uk/ccrl/4040/" rel="nofollow noreferrer">https://www.computerchess.org.uk/ccrl/4040/</a> an ELO ranking of many chess engines.</p>

<p>In the 2300-2600 range ("similar or a bit below a human master"), and amongst open-source engines, you have : </p>

<ul>
<li>K2 0.87 (2580 ELO)</li>
<li>Jazz 840 64-bit 4CPU (2552 ELO)</li>
<li>Maverick 1.5 64-bit (2522 ELO)</li>
<li>Winter 0.1.1 64-bit (2492 ELO)</li>
<li>Lozza 1.18 64-bit (2452 ELO)</li>
<li>Topple 0.1.1 64-bit (2360 ELO)</li>
</ul>

<p>I haven't tested these engines myself, but they seem to fit your criterias.</p>

<p><strong>Update</strong> : If you're looking more for engines in the 2000-2300 range, then these engines might suit you better : </p>

<ul>
<li>Absolute Zero 2.4.7.2 64-bit (ELO 2294)</li>
<li>CT800 1.30 64-bit (ELO 2253)</li>
<li>Zevra 1.8.4 r650 64-bit (ELO 2236)</li>
<li>ZCT 0.3.2451 (ELO 2210)</li>
<li>Tunguska 1.0 64-bit (ELO 2133)</li>
<li>Cinnamon 2.0 64-bit (ELO 2054)</li>
</ul>
 825: <p><a href="http://computer-chess.org/doku.php?id=computer_chess:wiki:lists:chess_engine_list" rel="nofollow noreferrer">http://computer-chess.org/doku.php?id=computer_chess:wiki:lists:chess_engine_list</a></p>

<ul>
<li>Arasan</li>
<li>GreKo</li>
<li>Crafty</li>
<li>Rodent</li>
<li>Toga etc</li>
</ul>
 11241: <p>Since your engine supports UCI, I would suggest checking out <a href="http://lichess.org" rel="noreferrer">lichess.org</a>. 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!!</p>

<p>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.</p>

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

<ol>
<li><a href="https://lichess.org/@/LeelaChess" rel="noreferrer">LeelaChess</a>: A neural net based BOT.</li>
<li><a href="https://lichess.org/@/UCIbot" rel="noreferrer">ASMFish</a></li>
</ol>

<p>There are many more BOTs on lichess, you can view them here on <a href="https://lichess.org/team/lichess-bots" rel="noreferrer">lichess BOT team</a>.</p>

<p>You can also find many not so strong self-implemented engines that you might like testing against, for example <a href="https://lichess.org/@/TrolliumBot" rel="noreferrer">this</a>.</p>
