fix: pin cython<3 on setup [backport #6376 to 1.15] (#6382)

Backports #6376 to 1.15.

## Checklist

- [x] Change(s) are motivated and described in the PR description.
- [x] Testing strategy is described if automated tests are not included
in the PR.
- [x] Risk is outlined (performance impact, potential for breakage,
maintainability, etc).
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[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist

- [x] Title is accurate.
- [x] No unnecessary changes are introduced.
- [x] Description motivates each change.
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[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes unless absolutely necessary.
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Author: Yun-Kim

pyproject.toml

@@ -1,5 +1,5 @@
 [build-system]
-requires = ["setuptools >= 40.6.0", "setuptools_scm[toml] >=4,<6.1", "cython"]
+requires = ["setuptools >= 40.6.0", "setuptools_scm[toml] >=4,<6.1", "cython<3"]
 build-backend = "setuptools.build_meta"
 
 [tool.setuptools_scm]

releasenotes/notes/pin-cython-042240c7d4fdd977.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    Pin ``cython<3`` due to an incompatibility with ``cython==3.0.0`` and typing annotations in
+    profiling code.

setup.py

@@ -491,7 +491,7 @@ def get_exts_for(name):
         "Programming Language :: Python :: 3.11",
     ],
     use_scm_version={"write_to": "ddtrace/_version.py"},
-    setup_requires=["setuptools_scm[toml]>=4", "cython"],
+    setup_requires=["setuptools_scm[toml]>=4", "cython<3"],
     ext_modules=ext_modules
     + cythonize(
         [