MNT: Set package name according to PEP-625

PyPI complained over email.

https://peps.python.org/pep-0625/

Author: kernc

setup.py

@@ -9,7 +9,7 @@
     from setuptools import setup, find_packages
 
     setup(
-        name='Backtesting',
+        name='backtesting',
         description="Backtest trading strategies in Python",
         license='AGPL-3.0',
         url='https://kernc.github.io/backtesting.py/',