Support resolving for an alternate Python distribution (#364)

## Summary

Low-priority but fun thing to end the day. You can now pass
`--target-version py37`, and we'll generate a resolution for Python 3.7.

See: https://github.com/astral-sh/puffin/issues/183.

crates/puffin-cli/tests/pip_compile.rs

@@ -554,6 +554,82 @@ optional-dependencies.foo = [
     Ok(())
 }
 
+/// Resolve a specific version of Black at Python 3.12.
+#[test]
+fn compile_python_312() -> Result<()> {
+    let temp_dir = assert_fs::TempDir::new()?;
+    let cache_dir = assert_fs::TempDir::new()?;
+    let venv = temp_dir.child(".venv");
+
+    Command::new(get_cargo_bin(BIN_NAME))
+        .arg("venv")
+        .arg(venv.as_os_str())
+        .arg("--cache-dir")
+        .arg(cache_dir.path())
+        .current_dir(&temp_dir)
+        .assert()
+        .success();
+    venv.assert(predicates::path::is_dir());
+
+    let requirements_in = temp_dir.child("requirements.in");
+    requirements_in.touch()?;
+    requirements_in.write_str("black==23.10.1")?;
+
+    insta::with_settings!({
+        filters => INSTA_FILTERS.to_vec()
+    }, {
+        assert_cmd_snapshot!(Command::new(get_cargo_bin(BIN_NAME))
+            .arg("pip-compile")
+            .arg("requirements.in")
+            .arg("--python-version")
+            .arg("py312")
+            .arg("--cache-dir")
+            .arg(cache_dir.path())
+            .env("VIRTUAL_ENV", venv.as_os_str())
+            .current_dir(&temp_dir));
+    });
+
+    Ok(())
+}
+
+/// Resolve a specific version of Black at Python 3.7.
+#[test]
+fn compile_python_37() -> Result<()> {
+    let temp_dir = assert_fs::TempDir::new()?;
+    let cache_dir = assert_fs::TempDir::new()?;
+    let venv = temp_dir.child(".venv");
+
+    Command::new(get_cargo_bin(BIN_NAME))
+        .arg("venv")
+        .arg(venv.as_os_str())
+        .arg("--cache-dir")
+        .arg(cache_dir.path())
+        .current_dir(&temp_dir)
+        .assert()
+        .success();
+    venv.assert(predicates::path::is_dir());
+
+    let requirements_in = temp_dir.child("requirements.in");
+    requirements_in.touch()?;
+    requirements_in.write_str("black==23.10.1")?;
+
+    insta::with_settings!({
+        filters => INSTA_FILTERS.to_vec()
+    }, {
+        assert_cmd_snapshot!(Command::new(get_cargo_bin(BIN_NAME))
+            .arg("pip-compile")
+            .arg("requirements.in")
+            .arg("--python-version")
+            .arg("py37")
+            .arg("--cache-dir")
+            .arg(cache_dir.path())
+            .env("VIRTUAL_ENV", venv.as_os_str())
+            .current_dir(&temp_dir));
+    });
+
+    Ok(())
+}
+
 /// Resolve a specific Flask wheel via a URL dependency.
 #[test]
 fn compile_wheel_url_dependency() -> Result<()> {

crates/puffin-cli/tests/snapshots/pip_compile__compile_python_312.snap

@@ -0,0 +1,34 @@
+---
+source: crates/puffin-cli/tests/pip_compile.rs
+info:
+  program: puffin
+  args:
+    - pip-compile
+    - requirements.in
+    - "--python-version"
+    - py312
+    - "--cache-dir"
+    - /var/folders/nt/6gf2v7_s3k13zq_t3944rwz40000gn/T/.tmpbKzceW
+  env:
+    VIRTUAL_ENV: /var/folders/nt/6gf2v7_s3k13zq_t3944rwz40000gn/T/.tmpZkKRNz/.venv
+---
+success: true
+exit_code: 0
+----- stdout -----
+# This file was autogenerated by Puffin v0.0.1 via the following command:
+#    [BIN_PATH] pip-compile requirements.in --python-version py312 --cache-dir [CACHE_DIR]
+black==23.10.1
+click==8.1.7
+    # via black
+mypy-extensions==1.0.0
+    # via black
+packaging==23.2
+    # via black
+pathspec==0.11.2
+    # via black
+platformdirs==3.11.0
+    # via black
+
+----- stderr -----
+Resolved 6 packages in [TIME]
+

crates/puffin-cli/tests/snapshots/pip_compile__compile_python_37.snap

@@ -0,0 +1,45 @@
+---
+source: crates/puffin-cli/tests/pip_compile.rs
+info:
+  program: puffin
+  args:
+    - pip-compile
+    - requirements.in
+    - "--python-version"
+    - py37
+    - "--cache-dir"
+    - /var/folders/nt/6gf2v7_s3k13zq_t3944rwz40000gn/T/.tmpQwHoBA
+  env:
+    VIRTUAL_ENV: /var/folders/nt/6gf2v7_s3k13zq_t3944rwz40000gn/T/.tmp1TmUIW/.venv
+---
+success: true
+exit_code: 0
+----- stdout -----
+# This file was autogenerated by Puffin v0.0.1 via the following command:
+#    [BIN_PATH] pip-compile requirements.in --python-version py37 --cache-dir [CACHE_DIR]
+black==23.10.1
+click==8.1.7
+    # via black
+importlib-metadata==6.8.0
+    # via click
+mypy-extensions==1.0.0
+    # via black
+packaging==23.2
+    # via black
+pathspec==0.11.2
+    # via black
+platformdirs==3.11.0
+    # via black
+tomli==2.0.1
+    # via black
+typing-extensions==4.8.0
+    # via
+    #   black
+    #   importlib-metadata
+    #   platformdirs
+zipp==3.17.0
+    # via importlib-metadata
+
+----- stderr -----
+Resolved 10 packages in [TIME]
+