Fix timezone comparison in DatetimeIndex._generate
This fix is just a band-aid-- a real fix will have to wait for PEP 431 to unite all the time zone implementations.

Also refine a bare `except:` in the same method.

Fixes #8616 

need to have the tests from the referenced issue
 Derf.  Will do.
 @ischwabacher can you revisit/rebase
 closing pls reopen if/when updated
