When running MySQL/MariaDB with charset utf8mb4, a character is 4 bytes. Hence a key created on a VARCHAR(255) takes 255 characters * 4 bytes/characters = 1020 bytes. However Innodb defaults to a maximum of 767 bytes for an index.
The extension fails to install on Debian Stretch which uses utf8mb4 as the default charset:
```
Query: CREATE TABLE `namespace_names` (
`ns_id` INT(8) NOT NULL DEFAULT '0',
`ns_name` VARCHAR(200) NOT NULL DEFAULT '',
`ns_default` tinyint(1) NOT NULL DEFAULT '0',
`ns_canonical` tinyint(1) DEFAULT NULL,
PRIMARY KEY (`ns_name`)
) ENGINE=InnoDB
Function: Wikimedia\Rdbms\Database::sourceFile( /workspace/src/extensions/SpecialNamespaces/namespace_names.sql )
Error: 1071 Specified key was too long; max key length is 767 bytes (localhost:/tmp/quibble-mysql-n27678zx/socket)
```