ENH: Add dtype argument to read_sql_query (GH10285)
- [x] closes #10285 
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
@jorisvandenbossche tagging you since you were involved in the original issue. Could you have a look at this imp and lmk whether this is what you had in mind? @jorisvandenbossche could you (or maybe someone else) have a look at this PR? Thanks @jreback could you (or maybe someone else) have a look at this PR? Thanks Updated PR. Only Linux py38_np_dev fails, but due to other reasons. Merged master, everything succeeds.

I also saw a similar enhancement request for dtypes for `read_sql`. Once we agree on this imp we can easily extend it to that function as well @jreback could you have another look? Will fix the merge conflict in the docs later today In #13049 it is proposed to add the `dtype` argument to `read_sql`. This can be done in similar manner as in this PR. lgtm ping on greenish as a followup :-> can type any `dtype=` args in the codebase @jreback CI is greenish,Travis failed due to other reasons

Will work on dtype follow up tomorrow. thanks @avinashpancham 

small comment for followon