Allow overriding config attrs in pip_parse-generated install_deps (#751)

diff --git a/examples/pip_parse_vendored/README.md b/examples/pip_parse_vendored/README.md
index 616e291..f53260a 100644
--- a/examples/pip_parse_vendored/README.md
+++ b/examples/pip_parse_vendored/README.md

@@ -9,3 +9,23 @@
 - requirements.in - human editable, expresses only direct dependencies and load-bearing version constraints
 - requirements.txt - lockfile produced by pip-compile or other means
 - requirements.bzl - the "parsed" version of the lockfile readable by Bazel downloader
+
+The `requirements.bzl` file contains baked-in attributes such as `python_interpreter_target` as they were specified in the original `pip_parse` rule. These can be overridden at install time by passing arguments to `install_deps`. For example: 
+
+```python
+# Register a hermetic toolchain
+load("@rules_python//python:repositories.bzl", "python_register_toolchains")
+
+python_register_toolchains(
+    name = "python39",
+    python_version = "3.9",
+)
+load("@python39//:defs.bzl", "interpreter")
+
+# Load dependencies vendored by some other ruleset.
+load("@some_rules//:py_deps.bzl", "install_deps")
+
+install_deps(
+    python_interpreter_target = interpreter,
+)
+```

diff --git a/examples/pip_parse_vendored/requirements.bzl b/examples/pip_parse_vendored/requirements.bzl
index 58c6e7b..33199b0 100644
--- a/examples/pip_parse_vendored/requirements.bzl
+++ b/examples/pip_parse_vendored/requirements.bzl

@@ -41,11 +41,13 @@
     name = requirement.split(" ")[0].split("=")[0]
     return _annotations.get(name)
 
-def install_deps():
+def install_deps(**whl_library_kwargs):
+    whl_config = dict(_config)
+    whl_config.update(whl_library_kwargs)
     for name, requirement in _packages:
         whl_library(
             name = name,
             requirement = requirement,
             annotation = _get_annotation(requirement),
-            **_config
+            **whl_config
         )

diff --git a/python/pip_install/extract_wheels/parse_requirements_to_bzl.py b/python/pip_install/extract_wheels/parse_requirements_to_bzl.py
index 5762cf5..d0abcac 100644
--- a/python/pip_install/extract_wheels/parse_requirements_to_bzl.py
+++ b/python/pip_install/extract_wheels/parse_requirements_to_bzl.py

@@ -158,13 +158,15 @@
             name = requirement.split(" ")[0].split("=")[0]
             return _annotations.get(name)
 
-        def install_deps():
+        def install_deps(**whl_library_kwargs):
+            whl_config = dict(_config)
+            whl_config.update(whl_library_kwargs)
             for name, requirement in _packages:
                 whl_library(
                     name = name,
                     requirement = requirement,
                     annotation = _get_annotation(requirement),
-                    **_config
+                    **whl_config
                 )
         """.format(
             all_requirements=all_requirements,