Merge pull request #9440 from blbradley/period-refactor-to-cython

PERF: Move Period class and related functions to Cython module

Author: jreback

doc/source/whatsnew/v0.16.0.txt

@@ -208,6 +208,7 @@ Performance
 - Performance and memory usage improvements in ``merge`` when key space exceeds ``int64`` bounds (:issue:`9151`)
 - Performance improvements in multi-key ``groupby`` (:issue:`9429`)
 - Performance improvements in ``MultiIndex.sortlevel`` (:issue:`9445`)
+- Cythonized ``Period`` (:issue:`9440`)
 
 Bug Fixes
 ~~~~~~~~~

pandas/lib.pxd

@@ -0,0 +1 @@
+cdef bint is_null_datetimelike(v)