Add check for compatible setuptools version.

In order to avoid a recursive dependency issue when parsing
the aiohttp version during install setuptools needs:
pypa/setuptools@c457e68

This implies we need setuptools 46.4.0 or newer.

CHANGES/6205.bugfix

@@ -0,0 +1 @@
+Add check for compatible ``setuptools`` version.

pyproject.toml

@@ -11,3 +11,9 @@ issue_format = "`#{issue} <https://github.com/aio-libs/aiohttp/issues/{issue}>`_
 test-command = ""
 # don't build PyPy wheels, install from source instead
 skip = "pp*"
+
+[build-system]
+requires = [
+    "setuptools>=46.4.0",
+]
+build-backend = "setuptools.build_meta"

setup.py

@@ -1,13 +1,18 @@
 import os
 import pathlib
 import sys
+from distutils.version import StrictVersion
 
-from setuptools import Extension, setup
+from setuptools import Extension, __version__ as setuptools_version, setup
 
 if sys.version_info < (3, 7):
     raise RuntimeError("aiohttp 4.x requires Python 3.7+")
 
 
+if StrictVersion(setuptools_version) < StrictVersion("46.4.0"):
+    raise RuntimeError("aiohttp 4.x requires setuptools 46.4.0+")
+
+
 NO_EXTENSIONS = bool(os.environ.get("AIOHTTP_NO_EXTENSIONS"))  # type: bool
 HERE = pathlib.Path(__file__).parent
 IS_GIT_REPO = (HERE / ".git").exists()