[BE]: Add PEP621 project section to pyproject.toml (#153055)

Follow up to @ezyang's PR #153020 , but better uses PEP621 to reduce redundant fields and pass through metadata better to uv, setuptools, poetry and other tooling.

* Enables modern tooling like uv sync and better support for tools like poetry.
* Also allows us to set project wide settings that are respected by linters and IDE (in this example we are able centralize the minimum supported python version).
* Currently most of the values are dynamically fetched from setuptools, eventually we can migrate all the statically defined values to pyproject.toml and they will be autopopulated in the setuptool arguments.
* This controls what additional metadata shows up on PyPi . Special URL Names are listed here for rendering on pypi: https://packaging.python.org/en/latest/specifications/well-known-project-urls/#well-known-labels

These also clearly shows us what fields will need to be migrated to pyproject.toml over time from setup.py per #152276. Static fields be fairly easy to migrate, the dynamically built ones like requirements are a bit more challenging.

Without this, `uv sync` complains:
```
error: No `project` table found in: `pytorch/pyproject.toml`
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/153055
Approved by: https://github.com/ezyang

pyproject.toml

@@ -1,3 +1,27 @@
+[project]
+name = "torch"
+requires-python = ">=3.9"
+license = {text = "BSD-3-Clause"}
+dynamic = [
+  "authors",
+  "classifiers",
+  "entry-points",
+  "dependencies",
+  "description",
+  "keywords",
+  "optional-dependencies",
+  "readme",
+  "scripts",
+  "version",
+]
+
+[project.urls]
+Homepage = "https://pytorch.org/"
+Documentation = "https://pytorch.org/docs/"
+Source = "https://github.com/pytorch/pytorch"
+Forum = "https://discuss.pytorch.org/"
+
+
 [build-system]
 requires = [
     # After 75.8.2 dropped dep disttools API. Please fix
@@ -18,8 +42,6 @@ build-backend = "setuptools.build_meta:__legacy__"
 
 [tool.black]
 line-length = 88
-target-version = ["py38"]
-
 
 [tool.isort]
 src_paths = ["caffe2", "torch", "torchgen", "functorch", "test"]
@@ -42,7 +64,6 @@ standard_library = ["typing_extensions"]
 
 
 [tool.ruff]
-target-version = "py39"
 line-length = 88
 src = ["caffe2", "torch", "torchgen", "functorch", "test"]