Use stem requirement from git especific commit

This is supposed to fix "RuntimeError: dictionary keys changed during iteration" bug.
Details in https://www.mail-archive.com/tor-bugs@lists.torproject.org/msg186952.html

requirements.txt

@@ -7,5 +7,5 @@ PySocks>=1.7.0
 requests>=2.22.0
 requests-futures>=1.0.0
 soupsieve>=1.9.2
-stem>=1.7.1
+-e git+https://git.torproject.org/stem.git@b5aecb7#egg=stem
 torrequest>=0.1.0