MNT: Bump minimum Python>=3.9

Author: kernc

CONTRIBUTING.md

@@ -33,7 +33,7 @@ first [fork the project]. Then:
 
     git clone git@github.com:YOUR_USERNAME/backtesting.py
     cd backtesting.py
-    pip3 install -e '.[doc,test,dev]'
+    pip install -e '.[doc,test,dev]'
 
 [fork the project]: https://help.github.com/articles/fork-a-repo/
 

setup.py

@@ -1,8 +1,8 @@
 import os
 import sys
 
-if sys.version_info < (3, 6):
-    sys.exit('ERROR: Backtesting.py requires Python 3.6+')
+if sys.version_info < (3, 9):
+    sys.exit('ERROR: Backtesting.py requires Python 3.9+')
 
 
 if __name__ == '__main__':
@@ -55,7 +55,7 @@
             ],
         },
         test_suite="backtesting.test",
-        python_requires='>=3.6',
+        python_requires='>=3.9',
         author='Zach Lûster',
         classifiers=[
             'Intended Audience :: Financial and Insurance Industry',