add pathlib to third-party

Summary:
We need a good `glob()` function for the upcoming `files:` field. It turns out
that git's native globbing doesn't work very well; ** can't match the root
directory for example. Pathlib has an excellent glob, but it's only
standardized in 3.4. Add pathlib to third-party, for compatibility with 3.2 and
3.3. Now we can start using it instead of `os.path` throughout the codebase.

Test Plan: `scripts/validate_third_party.sh`

Reviewers: sean

Differential Revision: https://phabricator.buildinspace.com/D29

peru.yaml

@@ -1,6 +1,7 @@
 imports:
     pyyaml: third-party
     docopt: third-party
+    pathlib: third-party
 
 # Use --force to have peru overwrite the checked in copy.
 # See third-party/.
@@ -14,3 +15,10 @@ git module docopt:
     rev: 940b1eb28175c0e4855a1e3b529cdf97309aa634  # 0.6.1
     build: mkdir out && cp docopt.py out/
     export: out
+
+# Pathlib is standard in Python 3.4, but we include it here for 3.2 and 3.3.
+hg module pathlib:
+    url: https://bitbucket.org/pitrou/pathlib
+    rev: 5740b7d56a3dffea6a606e098b316f959dfe774a  # 1.0
+    build: mkdir out && cp pathlib.py out/
+    export: out