BUG: GH11616 fixes timezone selection error
closes #11616 
closes #11682 

pls add a note in 0.18.0 bug fixes as well.
 pls add a test for #11682 which I believe this should fix as well
 These errors are occurring for different reasons. First error #11616 is in _possibly_downcast_to_dtype. It is not able to retrieve correct date from int64 object. hence throwing an exception that I have fixed by checking for timezone info and then converting to correct timezone.
#11682 is in _infer_dtype_from_scalar function as correctly pointed out by ajenkins-cargometrics . We are checking for 'tz' attribute . I have fixed that by replacing 'tzinfo' and added test case for that. 
Please review. 
 looks good. pls add 2 entries in bug fixes in whatsnew for 0.18.0
 Done. Please review.
 @jreback  One of the five [Build job](https://travis-ci.org/pydata/pandas/builds/93192293) is failing . I am not able to figure out the reason . Please help.
 merged via https://github.com/pydata/pandas/commit/e8382665d12dcf270f587cf76cd0e6d5a1dac184

thanks!
