test(bzlmod): use a universal requirements file for 3.9

Summary:
- separate target to update the pip requirements
- Use a universal requirements file

examples/BUILD.bazel

@@ -12,4 +12,14 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+# The following is experimental API and currently not intended for use outside this example.
+load("@rules_python//python/uv/private:pin.bzl", "pin")  # buildifier: disable=bzl-visibility
+
 licenses(["notice"])  # Apache 2.0
+
+pin(
+    name = "bzlmod_requirements_3_9_update",
+    srcs = ["bzlmod/requirements.in"],
+    out = "bzlmod/requirements_lock_3_9.txt",
+    python_version = "3.9.19",
+)

examples/bzlmod/BUILD.bazel

@@ -9,20 +9,10 @@ load("@bazel_skylib//rules:build_test.bzl", "build_test")
 load("@pip//:requirements.bzl", "all_data_requirements", "all_requirements", "all_whl_requirements", "requirement")
 load("@python_3_9//:defs.bzl", py_test_with_transition = "py_test")
 load("@python_versions//3.10:defs.bzl", compile_pip_requirements_3_10 = "compile_pip_requirements")
-load("@python_versions//3.9:defs.bzl", compile_pip_requirements_3_9 = "compile_pip_requirements")
 load("@rules_python//python:defs.bzl", "py_binary", "py_library", "py_test")
 
 # This stanza calls a rule that generates targets for managing pip dependencies
-# with pip-compile.
-compile_pip_requirements_3_9(
-    name = "requirements_3_9",
-    src = "requirements.in",
-    requirements_txt = "requirements_lock_3_9.txt",
-    requirements_windows = "requirements_windows_3_9.txt",
-)
-
-# This stanza calls a rule that generates targets for managing pip dependencies
-# with pip-compile.
+# with pip-compile for a particular python version.
 compile_pip_requirements_3_10(
     name = "requirements_3_10",
     timeout = "moderate",

examples/bzlmod/MODULE.bazel

@@ -135,17 +135,7 @@ pip.parse(
     ],
     hub_name = "pip",
     python_version = "3.9",
-    # The requirements files for each platform that we want to support.
-    requirements_by_platform = {
-        # Default requirements file for needs to explicitly provide the platforms
-        "//:requirements_lock_3_9.txt": "linux_*,osx_*",
-        # This API allows one to specify additional platforms that the users
-        # configure the toolchains for themselves. In this example we add
-        # `windows_aarch64` to illustrate that `rules_python` won't fail to
-        # process the value, but it does not mean that this example will work
-        # on Windows ARM.
-        "//:requirements_windows_3_9.txt": "windows_x86_64,windows_aarch64",
-    },
+    requirements_lock = "requirements_lock_3_9.txt",
     # These modifications were created above and we
     # are providing pip.parse with the label of the mod
     # and the name of the wheel.
@@ -179,8 +169,17 @@ pip.parse(
     ],
     hub_name = "pip",
     python_version = "3.10",
-    requirements_lock = "//:requirements_lock_3_10.txt",
-    requirements_windows = "//:requirements_windows_3_10.txt",
+    # The requirements files for each platform that we want to support.
+    requirements_by_platform = {
+        # Default requirements file for needs to explicitly provide the platforms
+        "//:requirements_lock_3_10.txt": "linux_*,osx_*",
+        # This API allows one to specify additional platforms that the users
+        # configure the toolchains for themselves. In this example we add
+        # `windows_aarch64` to illustrate that `rules_python` won't fail to
+        # process the value, but it does not mean that this example will work
+        # on Windows ARM.
+        "//:requirements_windows_3_10.txt": "windows_x86_64,windows_aarch64",
+    },
     # These modifications were created above and we
     # are providing pip.parse with the label of the mod
     # and the name of the wheel.