Avoid importing os unless necessary

Normally I wouldn't push the line on PEP 8 for importing `os`, but
`win32.lib.pywin32_bootstrap` is imported by `site` at Python startup via
pywin32.pth. Programs that care a lot about startup time might appreciate our
caution here.

win32/Lib/pywin32_bootstrap.py

@@ -6,14 +6,14 @@
 # modules are imported.
 # If Python has `os.add_dll_directory()`, we need to call it with this path.
 # Otherwise, we add this path to PATH.
-import os
 
 
 try:
     import pywin32_system32
 except ImportError:  # Python ≥3.6: replace ImportError with ModuleNotFoundError
     pass
 else:
+    import os
     # We're guaranteed only that __path__: Iterable[str]
     # https://docs.python.org/3/reference/import.html#__path__
     for path in pywin32_system32.__path__: