Drop support for Python 3.7

Author: spyoungtech

setup.py

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