fix various linter errors

lib/unittest.bzl

diff --git a/lib/unittest.bzl b/lib/unittest.bzl
index d459f0a..0d5307b 100644
--- a/lib/unittest.bzl
+++ b/lib/unittest.bzl
@@ -32,7 +32,7 @@
 
 TOOLCHAIN_TYPE = "@bazel_skylib//toolchains/unittest:toolchain_type"
 
-_UnittestToolchain = provider(
+_UnittestToolchainInfo = provider(
     doc = "Execution platform information for rules in the bazel_skylib repository.",
     fields = ["file_ext", "success_templ", "failure_templ", "join_on"],
 )
@@ -40,7 +40,7 @@
 def _unittest_toolchain_impl(ctx):
     return [
         platform_common.ToolchainInfo(
-            unittest_toolchain_info = _UnittestToolchain(
+            unittest_toolchain_info = _UnittestToolchainInfo(
                 file_ext = ctx.attr.file_ext,
                 success_templ = ctx.attr.success_templ,
                 failure_templ = ctx.attr.failure_templ,

tests/maprule_tests.bzl

diff --git a/tests/maprule_tests.bzl b/tests/maprule_tests.bzl
index 49a346d..5261d9a 100644
--- a/tests/maprule_tests.bzl
+++ b/tests/maprule_tests.bzl
@@ -45,7 +45,7 @@
             return
     unittest.fail(env, msg + ": did not find \"%s\" in: [%s]" % (expected_fragment, "\n".join(errors)))
 
-def _contains_substrings_in_order(env, s, substrings):
+def _contains_substrings_in_order(s, substrings):
     index = 0
     for ss in substrings:
         index = s.find(ss, index)
@@ -56,7 +56,7 @@
 
 def _assert_error_fragments(env, errors, expected_fragments, msg):
     for e in errors:
-        if _contains_substrings_in_order(env, e, expected_fragments):
+        if _contains_substrings_in_order(e, expected_fragments):
             return
     unittest.fail(env, msg + ": did not find expected fragments in \"%s\" in order" % "\n".join(errors))
 
@@ -337,7 +337,7 @@
         src3 = _mock_file(ctx, language + "/bar/src1.txt")
         foreach_srcs = [src1, src2, src3]
 
-        outs_dicts, all_output_files, src_placeholders_dicts, errors = (
+        outs_dicts, all_output_files, _, errors = (
             maprule_testing.create_outputs(
                 ctx,
                 "my_maprule",

workspace.bzl

diff --git a/workspace.bzl b/workspace.bzl
index 50b6db5..6bf67b5 100644
--- a/workspace.bzl
+++ b/workspace.bzl
@@ -1,3 +1,5 @@
+"""Dependency registration helpers for repositories which need to load bazel-skylib."""
+
 load("@bazel_skylib//lib:unittest.bzl", "register_unittest_toolchains")
 
 def bazel_skylib_workspace():