Pyathena

* Fix Pandas to lazy import.
Thanks sroussi. (laughingman7743/PyAthena/issues/82)

* Support integer NA values in PnadasCursor.
Restrict the allowable version of Pandas(>=0.24.0).
Thanks austinlostinboston, mckeown12 and xinluo-gogovan. (laughingman7743/PyAthena/issues/60)
* Drop Python 3.4 support.
It will work with Python 3.4 unless you use PandasCusrsor.

* Support Python 3.7.

* Allow s3_staging_dir to be overridden in query execution.
Thanks aidin36. (laughingman7743/PyAthena/issues/53)
* Support work_group parameter in query execution.
Thanks sharath-thirunagaru. (laughingman7743/PyAthena/issues/69)
* Fix cursor constructor argument to be overridden by cursor method kwargs.
* Aggregate retry settings into RetryConfig object.
* Restrict the allowable version of SQLAlchemy(>=1.0.0, <1.3.0).

* Support botocore config argument.
Thanks m-for-madness. (laughingman7743/PyAthena/issues/64)

* Fix AttributeError: 'Engine' object has no attribute 'connection'.
Thanks mrshu. (laughingman7743/PyAthena/issues/63)