[build] Remove python venv_bin filegroup

The filegroup unnecessarily couples our BUILD files to our repository
rule, without the py_toolchain as an intermediary. Our goal is to use
the toolchain to fully encapsulate the selected python interpreter.

Previously the venv_bin had only two citations:

(1) Jupyter executable lookup from a jupyter regression test.

The test had no need for a subprocess; the production code that
it's guarding (jupyter_bazel) imports jupyter from sys.path, it
doesn't shell out. We rework the test to better match what it's
guarding, and therefore can drop the venv_bin dependency.

(2) Python interpreter lookup from installer regression test.

The sys.executable used for testing already points to the venv python.
The Rlocation lookup was vestigial, from an earlier draft that added
the venv libraries with deps instead of with the toolchain.

tools/install/BUILD.bazel

@@ -26,10 +26,7 @@ drake_py_library(
     name = "install_test_helper",
     testonly = 1,
     srcs = ["install_test_helper.py"],
-    data = [
-        "//:install",
-        "@python//:venv_bin",
-    ],
+    data = ["//:install"],
     imports = ["."],
     deps = [
         "@rules_python//python/runfiles",

tools/install/install_test.py

@@ -29,14 +29,8 @@ def test_basic_paths(self):
         self.assertSetEqual(set(['bin', 'include', 'lib', 'share']), content)
 
     def _run_one_command(self, test_command):
-        # On macOS, we must run the install tests against venv's interpreter,
-        # so that necessary libraries are available. On Ubuntu, the libraries
-        # are installed system-wide (without a venv).
-        if sys.platform == "darwin":
-            manifest = runfiles.Create()
-            python = manifest.Rlocation("python/bin/python3")
-        else:
-            python = install_test_helper.get_python_executable()
+        assert test_command.endswith(".py")
+        python = install_test_helper.get_python_executable()
 
         # Our launched processes should be independent, not inherit their
         # runfiles from the install_test.py runner.
@@ -47,7 +41,6 @@ def _run_one_command(self, test_command):
 
         # Execute the test_command.
         print("+ {}".format(test_command), file=sys.stderr)
-        assert test_command.endswith(".py")
         subprocess.check_call(
             [python, os.path.join(os.getcwd(), test_command)],
             env=env)

tools/jupyter/BUILD.bazel

@@ -31,11 +31,7 @@ drake_jupyter_py_binary(
 )
 
 drake_py_unittest(
-    name = "jupyter_notebook_test",
-    data = ["@python//:venv_bin"],
-    deps = [
-        "@rules_python//python/runfiles",
-    ],
+    name = "jupyter_prereqs_test",
 )
 
 # Add example failing notebook.

tools/jupyter/test/jupyter_prereqs_test.py

@@ -0,0 +1,14 @@
+import unittest
+
+from jupyter_core.command import list_subcommands
+
+
+class TestJupyterPrereqs(unittest.TestCase):
+    """Sanity checks that our install_prereqs was sufficient."""
+
+    def test_notebook(self):
+        """Ensures that `jupyter notebook` is installed (#12042). That
+        subcommand is required by _jupyter_bazel_notebook_main when running
+        interactively (i.e., when in non-test mode).
+        """
+        self.assertIn("notebook", list_subcommands())

tools/workspace/python/package.BUILD.bazel

@@ -44,12 +44,3 @@ cc_library(
 )
 
 exports_files(["requirements.txt"])
-
-filegroup(
-    name = "venv_bin",
-    visibility = ["//visibility:public"],
-    data = [
-        "bin",
-        "requirements.txt",
-    ],
-)

tools/workspace/python/repository.bzl

@@ -13,8 +13,7 @@ dynamic libraries to be used as Python modules.
 interpreter as a library into an executable with a C++ main() function.
 
 For part (b) the environment is used in all python rules and tests by default,
-but if a test needs to shell out to a venv binary, the `@python//:venv_bin`
-can be used to put the binaries' path into runfiles.
+because the python toolchain's interpreter is the venv python3 binary.
 
 If the {macos,linux}_interpreter_path being used only mentions the python major
 version (i.e., it is "/path/to/python3" not "/path/to/python3.##") and if the