Removed 3.6 from the pyenv versions in maintainers_guide.md

Author: ewanek1

.github/maintainers_guide.md

@@ -25,7 +25,6 @@ $ pyenv local 3.8.5
 
 $ pyenv versions
   system
-  3.6.10
   3.7.7
 * 3.8.5 (set by /path-to-bolt-python/.python-version)