feat: assert that Bazel is at least 4.0.0 LTS

Since we only test against bazel 4.0 now, and will start using features that require it, this gives users
an obvious error message that they need to update.

python/pip_install/repositories.bzl

@@ -1,5 +1,6 @@
 ""
 
+load("@bazel_skylib//lib:versions.bzl", "versions")
 load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
 load("@bazel_tools//tools/build_defs/repo:utils.bzl", "maybe")
 
@@ -63,6 +64,13 @@ def pip_install_dependencies():
 
     (However we call it from pip_install, making it optional for users to do so.)
     """
+
+    # We only support Bazel LTS and rolling releases.
+    # Give the user an obvious error to upgrade rather than some obscure missing symbol later.
+    # It's not guaranteed that users call this function, but it's used by all the pip fetch
+    # repository rules so it's likely that most users get the right error.
+    versions.check("4.0.0")
+
     for (name, url, sha256) in _RULE_DEPS:
         maybe(
             http_archive,