Fix the code to abbreviate Python's version (#2618)

This breaks for Python >= 3.10

Co-authored-by: Álvaro Mondéjar Rubio <mondejar1994@gmail.com>

Author: oprypin

mkdocs/__main__.py

@@ -146,7 +146,7 @@ def callback(ctx, param, value):
     click.option('--use-directory-urls/--no-directory-urls', is_flag=True, default=None, help=use_directory_urls_help)
 ])
 
-PYTHON_VERSION = sys.version[:3]
+PYTHON_VERSION = f"{sys.version_info.major}.{sys.version_info.minor}"
 
 PKG_DIR = os.path.dirname(os.path.abspath(__file__))