Actually fix our docs build

pyproject.toml

@@ -45,21 +45,16 @@ pyopenssl = ">=0.13"
 # For pkg_resources. >=1.0 so pip resolves it to a version cryptography
 # will tolerate; see #2599:
 setuptools = ">=1.0"
-# Our sphinx dependency is currently specified in this way to avoid making our
-# version requirement overly strict for people on older versions of Python.
-sphinx = [
- # >=1.0 is needed for autodoc_member_order = 'bysource', autodoc_default_flags
- {version = ">=1.0", optional = true},
- # >=4.2.0 is needed for Python 3.10 support. See
- # https://github.com/sphinx-doc/sphinx/issues/9816.
- {version = ">=4.2.0", optional = true, python = ">=3.10"}
-]
+# >=4.3.0 is needed for Python 3.10 support
+sphinx = {version = ">=4.3.0", optional = true}
 sphinx-rtd-theme = {version = ">=1.0", optional = true}
 
 [tool.poetry.dev-dependencies]
 # coverage[toml] extra is required to read the coverage config from pyproject.toml
 coverage = {version = ">=4.0", extras = ["toml"]}
-flake8 = "*"
+# flake8 3.x doesn't work on Python 3.7. See
+# https://github.com/PyCQA/flake8/issues/1701.
+flake8 = ">=4.0"
 flake8-assertive = "*"
 isort = "*"
 mypy = "*"