Sqlalchemy-exasol

* Fixed bug when accessing underlying odbc connection while using NullPool based engine
(Note: this addresses the superset [issue-20105](https://github.com/apache/superset/issues/20105))
* Remove odbc specific functionality from base dialect
* Removed remaining python2 compatibility artifacts and switches
* Bump support of base python version to 3.7

* Update supported versions of EXASOL DB to 7.1.6 and 7.0.16
* Update supported python versions to 3.6, 3.7, 3.8, 3.9
* bumped SQLAlchemy dependency to 1.3.24
* bumped pyodbc dependency to 4.0.32
* Fixed bug regarding maximum identifier length
* Fixed bug with custom translate maps
* Fixed bug regarding non existent error code mappings for EXASOL specific odbc error codes
* Updated documentation to reflect the latest version changes etc.
* Updated maintainer and contact information
* Removed outdated documentation and resources

* dropped EXASOL 5 from integration tests
* bumped to SQLAlchemy 1.2.8
* Support the turbodbc parameters `varchar_max_character_limit`, `prefer_unicode`,
`large_decimals_as_64_bit_types`, and `limit_varchar_results_to_max`.

upgrade to sqlalchemy 1.0.x (incompatible changes drop support for older sqlalchemy versions)

See CHANGES.md