Pyexasol

- Added `websocket_sslopt` connection option, to set custom SSL options for WebSocket connection. See [WebSocket client code](https://github.com/websocket-client/websocket-client/blob/2222f2c49d71afd74fcda486e3dfd14399e647af/websocket/_http.py#L210-L272) for more details.
- Add a basic benchmark to compare performance of individual nodes. Documentation will be added shortly.

- Run Travis tests with lowest (3.6) and highest (3.9) supported Python versions only.
- Updated description and classifiers for PyPi.

- Fixed the problem with `delimit` HTTP transport parameter expecting `NONE` value instead of `NEVER`.

ExaConnection

- Added [`protocol_version`](/docs/REFERENCE.mdconnect) connection option to adjust the protocol version requested by client (default: `pyexasol.PROTOCOL_V1`).
- Added [`.protocol_version()`](/docs/REFERENCE.mdprotocol_version) function to check the actual protocol version of established connection.

ExaMetaData

- Added [`.meta.execute_meta_nosql()`](/docs/REFERENCE.mdexecute_meta_nosql) function to run [no SQL metadata commands](https://github.com/exasol/websocket-api/blob/master/docs/WebsocketAPIV2.md#metadata-related-commands) introduced in Exasol v7.0+.
- Function [`.meta.execute_snapshot()`](/docs/REFERENCE.mdexecute_snapshot) is not public. You may use it run complex metadata SQL queries in snapshot isolation mode.

ExaStatement

- Added ability to execute no SQL metadata commands AND process the response as normal SQL-like result set. It does not change anything in public interface, but it might have an impact if you use custom overloaded `ExaStatement` class.

- Re-throw `BrokenPipeError` (and other sub-classes of `ConnectionError`) as `ExaCommunicationError`. This type of errors might not be handled in WebSocket client library in certain cases.

- Added optional [`disconnect`](https://github.com/exasol/websocket-api/blob/master/WebsocketAPI.md#disconnect-closes-a-connection-to-exasol) command executed during `.close()`. It is now enabled by default , but can be disabled with explicit `.close(disconnect=False)` to revert to original behaviour;
- Added `csv_cols` to [HTTP transport parameters](/docs/HTTP_TRANSPORT.mdparameters). It allows to skip some columns in CSV and adjust numeric and date format during IMPORT and EXPORT. It is still recommended to implement your own data transformation layer, since `csv_cols` capabilities are limited;