Add support for specifying python_platform in pip_compile (#16)

uv 0.1.35 added the `--python-platform` option to `uv pip compile`:

```
      --python-platform <PYTHON_PLATFORM>
          The platform for which requirements should be resolved.

          Represented as a "target triple", a string that describes the target platform in terms of its CPU, vendor, and operating system
          name, like `x86_64-unknown-linux-gnu` or `aaarch64-apple-darwin`.

          Possible values:
          - windows:                    An alias for `x86_64-pc-windows-msvc`, the default target for Windows
          - linux:                      An alias for `x86_64-unknown-linux-gnu`, the default target for Linux
          - macos:                      An alias for `aarch64-apple-darwin`, the default target for macOS
          - x86_64-pc-windows-msvc:     An x86 Windows target
          - x86_64-unknown-linux-gnu:   An x86 Linux target
          - aarch64-apple-darwin:       An ARM-based macOS target, as seen on Apple Silicon devices
          - x86_64-apple-darwin:        An x86 macOS target
          - aarch64-unknown-linux-gnu:  An ARM64 Linux target
          - aarch64-unknown-linux-musl: An ARM64 Linux target
          - x86_64-unknown-linux-musl:  An x86_64 Linux target
```

In this PR, enable specifying any (or none) of these values using the `python_platform` arg in `pip_compile`.

examples/typical/BUILD.bazel

@@ -3,4 +3,10 @@ load("@rules_uv//uv:venv.bzl", "create_venv")
 
 pip_compile(name = "generate_requirements_txt")
 
+pip_compile(
+    name = "generate_requirements_linux_txt",
+    python_platform = "x86_64-unknown-linux-gnu",
+    requirements_txt = "requirements_linux.txt",
+)
+
 create_venv(name = "create-venv")

examples/typical/requirements_linux.txt

@@ -0,0 +1,3 @@
+click==8.1.7 \
+    --hash=sha256:ae74fb96c20a0277a1d615f1e4d73c8414f5a98db8b799a7931d1582f3390c28 \
+    --hash=sha256:ca9853ad459e787e2192211578cc907e7594e294c7ccc834310722b41b9ca6de

uv/private/pip.bzl

@@ -5,9 +5,15 @@ _PY_TOOLCHAIN = "@bazel_tools//tools/python:toolchain_type"
 _common_attrs = {
     "requirements_in": attr.label(mandatory = True, allow_single_file = True),
     "requirements_txt": attr.label(mandatory = True, allow_single_file = True),
+    "python_platform": attr.string(default = ""),
     "_uv": attr.label(default = "@multitool//tools/uv", executable = True, cfg = "exec"),
 }
 
+def _python_platform(maybe_python_platform):
+    if maybe_python_platform == "":
+        return ""
+    return "--python-platform {python_platform}".format(python_platform = maybe_python_platform)
+
 def _uv_pip_compile(ctx, template, executable):
     py_toolchain = ctx.toolchains[_PY_TOOLCHAIN]
     ctx.actions.expand_template(
@@ -18,6 +24,7 @@ def _uv_pip_compile(ctx, template, executable):
             "{{requirements_in}}": ctx.file.requirements_in.short_path,
             "{{requirements_txt}}": ctx.file.requirements_txt.short_path,
             "{{resolved_python}}": py_toolchain.py3_runtime.interpreter.short_path,
+            "{{python_platform}}": _python_platform(ctx.attr.python_platform),
         },
     )
 
@@ -64,18 +71,20 @@ _pip_compile_test = rule(
     test = True,
 )
 
-def pip_compile(name, requirements_in = None, requirements_txt = None, target_compatible_with = None):
+def pip_compile(name, requirements_in = None, requirements_txt = None, target_compatible_with = None, python_platform = None):
     _pip_compile(
         name = name,
         requirements_in = requirements_in or "//:requirements.in",
         requirements_txt = requirements_txt or "//:requirements.txt",
+        python_platform = python_platform or "",
         target_compatible_with = target_compatible_with,
     )
 
     _pip_compile_test(
         name = name + "_diff_test",
         requirements_in = requirements_in or "//:requirements.in",
         requirements_txt = requirements_txt or "//:requirements.txt",
+        python_platform = python_platform or "",
         target_compatible_with = target_compatible_with,
         tags = ["requires-network"],
     )

uv/private/pip_compile.sh

@@ -3,6 +3,7 @@
 set -euo pipefail
 
 UV="{{uv}}"
+PYTHON_PLATFORM="{{python_platform}}"
 RESOLVED_PYTHON="{{resolved_python}}"
 REQUIREMENTS_IN="{{requirements_in}}"
 REQUIREMENTS_TXT="{{requirements_txt}}"
@@ -20,6 +21,7 @@ $UV pip compile \
     --no-header \
     --no-strip-extras \
     --python-version=$PYTHON_VERSION \
+    $(echo $PYTHON_PLATFORM) \
     -o $REQUIREMENTS_TXT \
     $REQUIREMENTS_IN \
     $@

uv/private/pip_compile_test.sh

@@ -3,6 +3,7 @@
 set -euo pipefail
 
 UV="{{uv}}"
+PYTHON_PLATFORM="{{python_platform}}"
 RESOLVED_PYTHON="{{resolved_python}}"
 REQUIREMENTS_IN="{{requirements_in}}"
 REQUIREMENTS_TXT="{{requirements_txt}}"
@@ -21,5 +22,6 @@ $UV pip compile \
     --no-header \
     --no-strip-extras \
     --python-version=$PYTHON_VERSION \
+    $(echo $PYTHON_PLATFORM) \
     -c $REQUIREMENTS_TXT \
     $REQUIREMENTS_IN