Convert the legacy [tool.flit.metadata] table to the standard [project] table.
flit_core 4 removed support for the old table, so the package fails to build
with any flit_core newer than 3.x.

diff --git a/pyproject.toml b/pyproject.toml
index e90693b..564bbf6 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -1,21 +1,21 @@
 [build-system]
-requires = ["flit_core <4"]
+requires = ["flit_core >=3.9,<5"]
 build-backend = "flit_core.buildapi"
 
-[tool.flit.metadata]
-module = "gidgethub"
-author = "Brett Cannon"
-author-email = "brett@python.org"
-requires = [
+[project]
+name = "gidgethub"
+authors = [{name = "Brett Cannon", email = "brett@python.org"}]
+readme = "README.rst"
+license = "Apache-2.0"
+license-files = ["LICENSE"]
+keywords = ["github", "sans-io", "async"]
+requires-python = ">=3.8"
+dependencies = [
     "uritemplate>=3.0.1",
-    "PyJWT[crypto]>=2.4.0"
+    "PyJWT[crypto]>=2.4.0",
 ]
-requires-python = ">=3.8"
-license = "Apache"
-keywords = "github sans-io async"
-description-file = "README.rst"
-classifiers = ["Intended Audience :: Developers",
-    "License :: OSI Approved :: Apache Software License",
+classifiers = [
+    "Intended Audience :: Developers",
     "Development Status :: 5 - Production/Stable",
     "Programming Language :: Python :: 3 :: Only",
     "Programming Language :: Python :: 3.8",
@@ -25,8 +25,9 @@ classifiers = ["Intended Audience :: Developers",
     "Programming Language :: Python :: 3.12",
     "Programming Language :: Python :: 3.13",
 ]
+dynamic = ["version", "description"]
 
-[tool.flit.metadata.requires-extra]
+[project.optional-dependencies]
 test = ["importlib-resources", "pytest>=5.4.1", "pytest-asyncio", "pytest-tornasync"]
 doc = ["sphinx>=4.0.0", "sphinx-rtd-theme>=0.5.2"]
 # The 'dev' target should contain all dependencies listed in any extras coming
@@ -37,7 +38,7 @@ aiohttp = ["aiohttp"]
 tornado = ["tornado"]
 httpx = ["httpx>=0.16.1"]
 
-[tool.flit.metadata.urls]
+[project.urls]
 Documentation = "https://gidgethub.readthedocs.io"
 Repository = "https://github.com/brettcannon/gidgethub"