Drop 3.7 support from install script

Resolves #232

newsfragments/232.removal

@@ -0,0 +1 @@
+Drop Python 3.7 support in preparation for supporting 3.9+ syntax.

setup.py

@@ -54,7 +54,7 @@ setup(
 
     ],
     tests_require=['pytest'],
-    python_requires=">=3.7",
+    python_requires=">=3.8",
     keywords=[
         'trio',
         "async",
@@ -71,7 +71,6 @@ setup(
         "License :: OSI Approved :: GNU General Public License v3 or later (GPLv3+)",
         "Programming Language :: Python :: Implementation :: CPython",
         "Programming Language :: Python :: 3 :: Only",
-        "Programming Language :: Python :: 3.7",
         "Programming Language :: Python :: 3.8",
         "Programming Language :: Python :: 3.9",
         "Intended Audience :: Science/Research",