Fix server version parsing (non-numeric build)

[carlosefr]

The server version handling code is assuming all version components are numeric, which isn't the case when the fourth component is a build tag (such as 12-stable-98abc83). This causes an exception when connecting to servers.

This PR keeps version components as a string when they can't be parsed as an integer (although I feel the proper course of action here would be returning the unparsed version in the tuple, and then use distutils.version.LooseVersion for comparisons in ClickHouseDialect.initialize().

[coveralls]

Coverage remained the same at 95.511% when pulling 141c8c5 on carlosefr:fix_version_check into 676034c on xzkostyan:master.