unpin uvloop (#959)

Now that uvloop sets python_requires the pinning isn't needed, but we do need to
ignore the packages with missing python_requires

MagicStack/uvloop#401

setup.py

@@ -53,8 +53,7 @@ def get_packages(package):
 extra_requirements = [
     "websockets==8.*",
     "httptools==0.1.* ;" + env_marker_cpython,
-    "uvloop==0.14.0; python_version == '3.6' and " + env_marker_cpython,
-    "uvloop>=0.14.0; python_version >= '3.7' and " + env_marker_cpython,
+    "uvloop>=0.14.0,!=0.15.0,!=0.15.1; " + env_marker_cpython,
     "colorama>=0.4;" + env_marker_win,
     "watchgod>=0.6",
     "python-dotenv>=0.13",