Python templater

Uses native Python f-strings. As described in Generic Variable Templating, an example usage would look be configured as follows:

If passed the following .sql file:

SELECT * FROM {tbl_name}

…and the following configuration in .sqlfluff in the same directory:

[sqlfluff]
templater = python

[sqlfluff:templater:python:context]
tbl_name = my_table

…then before parsing, the sql will be transformed to:

SELECT * FROM my_table