support python >=3.7,<4.0.0

pyproject.toml

@@ -35,7 +35,7 @@ classifiers = [
 readme='README.rst'
 
 [tool.poetry.dependencies]
-python = "^3.8"
+python = ">=3.7,<4.0.0"
 colorama = "*"  # Console colors under windows: https://pypi.org/project/colorama/
 colorlog = "*"  # https://github.com/borntyping/python-colorlog
 gunicorn = "*"  # https://gunicorn.org/