ENH: always register our Arrow extension types on pandas import
When initially adding the arrow extension types for Period and Interval (https://github.com/pandas-dev/pandas/pull/28371), there was some discussion about only importing pyarrow + registering those types lazily (on first usage), or eagerly on first "import pandas".  
I think we agreed that doing it eagerly (but of course still behind a try/except, since it's an optional dependency) was fine, since most of the import time of pyarrow is due to import numpy, which is something we need to do anyway. 

However, at the time that gave circular import problems with old pyarrow versions(https://github.com/pandas-dev/pandas/pull/28371#issuecomment-572032856). Nowadays we require more recent version of pyarrow, and this should no longer be a problem. 

(triggered by https://issues.apache.org/jira/browse/ARROW-12732)