tests: Add scan_code_imports benchmark

```console
$ ./tests/bench/scan_code
scan_code_imports python2.7  100 loops, best of 3: 3.19 msec per loop
scan_code_imports python3.9  500 loops, best of 5: 685 usec per loop
scan_code_imports python3.10  500 loops, best of 5: 727 usec per loop
scan_code_imports python3.11  500 loops, best of 5: 601 usec per loop
scan_code_imports python3.12  500 loops, best of 5: 609 usec per loop
scan_code_imports python3.13  500 loops, best of 5: 586 usec per loop
```

tests/bench/scan_code

@@ -0,0 +1,18 @@
+#!/usr/bin/env bash
+
+set -o errexit
+set -o pipefail
+set -o nounset
+
+BENCH_FUNC="scan_code_imports"
+BENCH_EXPR="deque($BENCH_FUNC(co), maxlen=0)"
+BIG_MODULE_PATH="$(dirname -- "$0")/data/big_module.py"
+IMPORTS="from collections import deque; from mitogen.master import $BENCH_FUNC"
+COMPILE="co=compile(open('$BIG_MODULE_PATH').read(), '$BIG_MODULE_PATH', 'exec')"
+PYTHONS=(
+    python2.7 python3.9 python3.10 python3.11 python3.12 python3.13
+)
+for p in "${PYTHONS[@]}"; do
+    echo -e -n "$BENCH_FUNC $p  "
+    $p -m timeit -s "$IMPORTS" -s "$COMPILE" "$BENCH_EXPR"
+done