tests: treat `select` as a built-in module on Windows

This fixes:

    --- C:/Users/Matt/hg/tests/test-check-module-imports.t
    +++ C:/Users/Matt/hg/tests/test-check-module-imports.t.err
    @@ -43,3 +43,15 @@
       > -X tests/test-verify-repo-operations.py \
       > -X tests/test-extension.t \
       > | sed 's-\\-/-g' | "$PYTHON" "$import_checker" -
    +  hgext/zeroconf/Zeroconf.py:86: stdlib import "socket" follows local import: select\r (esc)
    +  hgext/zeroconf/Zeroconf.py:87: stdlib import "struct" follows local import: select\r (esc)
    +  hgext/zeroconf/Zeroconf.py:88: stdlib import "threading" follows local import: select\r (esc)
    +  hgext/zeroconf/Zeroconf.py:89: stdlib import "time" follows local import: select\r (esc)
    +  hgext/zeroconf/Zeroconf.py:90: stdlib import "traceback" follows local import: select\r (esc)
    +  mercurial/posix.py:18: stdlib import "stat" follows local import: select\r (esc)
    +  mercurial/posix.py:19: stdlib import "sys" follows local import: select\r (esc)
    +  mercurial/posix.py:20: stdlib import "tempfile" follows local import: select\r (esc)
    +  mercurial/posix.py:21: stdlib import "typing" follows local import: select\r (esc)
    +  tests/tinyproxy.py:19: stdlib import "socket" follows local import: select\r (esc)
    +  tests/tinyproxy.py:20: stdlib import "sys" follows local import: select\r (esc)
    +  [1]

    ERROR: test-check-module-imports.t output changed

contrib/import-checker.py

@@ -231,7 +231,7 def list_stdlib_modules():
     yield 'importlib.machinery'  # python3 only
     yield 'importlib.util'  # python3 only
     yield 'packaging.version'
-    for m in 'fcntl', 'grp', 'pwd', 'termios':  # Unix only
+    for m in 'fcntl', 'grp', 'pwd', 'select', 'termios':  # Unix only
         yield m
     for m in 'cPickle', 'datetime':  # in Python (not C) on PyPy
         yield m