Fix flake8 with Python 2.7

Summary:
flake8 (in Python 2.7 mode) complains that `typing` is mentioned in type annotations but is not defined:

```
fbcode_builder/getdeps/buildopts.py:251:21: F821 undefined name 'typing'
    subst_mapping,  # type: typing.Mapping[str, str]
                    ^
fbcode_builder/getdeps/buildopts.py:253:5: F821 undefined name 'typing'
    # type: (...) -> typing.Optional[str]
    ^
2     F821 undefined name 'typing'
2
```

Import `typing` explicitly to silence this warning.

Because `typing` may be unavailable, import it conditionally. (Because it's only referenced in comments, failing to import `typing` should have no effect at run time.)

Reviewed By: snarkmaster

Differential Revision: D16435696

fbshipit-source-id: 78a4a7b07acc46aa998f02b54b1a6e52c1daafde

build/fbcode_builder/getdeps/buildopts.py

@@ -21,6 +21,12 @@ from .envfuncs import Env, add_path_entry, path_search
 from .platform import HostType, is_windows
 
 
+try:
+    import typing  # noqa: F401
+except ImportError:
+    pass
+
+
 def containing_repo_type(path):
     while True:
         if os.path.exists(os.path.join(path, ".git")):