Update README

Signed-off-by: Tsuyoshi Hombashi <tsuyoshi.hombashi@gmail.com>

Author: thombashi

README.rst

@@ -227,7 +227,7 @@ Installation: dpkg (Debian/Ubuntu)
 
 Dependencies
 ============
-- Python 3.7+
+- Python 3.9+
 - `Python package dependencies (automatically installed) <https://github.com/thombashi/tcconfig/network/dependencies>`__
 
 Linux packages