Fix python_requires value

Author: pquentin

setup.py

@@ -19,7 +19,7 @@
  package_dir={"": "src"},
  install_requires=["tokenize_rt", "setuptools"],
  keywords=["async"],
- python_requires=">=3.7",
+ python_requires=">=3.8",
  classifiers=[
  "License :: OSI Approved :: MIT License",
  "License :: OSI Approved :: Apache Software License",