tools: Use root-based absolute import for tools.lib, etc.

Mypy can’t follow absolute imports based on directories other than the
root.  This was hiding some type errors due to ignore_missing_imports.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

tools/test-backend

@@ -1,10 +1,5 @@
 #!/usr/bin/env python3
 
-# check for the venv
-from lib import sanity_check
-
-sanity_check.check_venv(__file__)
-
 import argparse
 import contextlib
 import glob
@@ -16,6 +11,15 @@ import tempfile
 from typing import Iterator, List
 from unittest import mock
 
+TOOLS_DIR = os.path.dirname(os.path.abspath(__file__))
+os.chdir(os.path.dirname(TOOLS_DIR))
+sys.path.insert(0, os.path.dirname(TOOLS_DIR))
+
+# check for the venv
+from tools.lib import sanity_check
+
+sanity_check.check_venv(__file__)
+
 import django
 import orjson
 import responses
@@ -185,10 +189,6 @@ class ZulipInternetBlockedError(Exception):
 
 
 def main() -> None:
-    TOOLS_DIR = os.path.dirname(os.path.abspath(__file__))
-    os.chdir(os.path.dirname(TOOLS_DIR))
-    sys.path.insert(0, os.path.dirname(TOOLS_DIR))
-
     default_parallel = os.cpu_count()
 
     # Remove proxy settings for running backend tests