Scrapy-zyte-smartproxy

- Remove crawlera X-headers when crawlera is disabled
- Introduction of DEFAULT_CRAWLERA_HEADERS settings

- Recommend middleware order to be 610 to run before `RedirectMiddleware`.
- Change default download timeout to 190s or 3 minutes 10 seconds (instead of 1800s or 30 minutes).
- Test and advertize Python 3 compatiblity.
- New `crawlera/request` and `crawlera/request/method/*` stats counts.
- Clear Scrapy DNS cache for proxy URL in case of connection errors.
- Distribute plugin as universal wheel.