Copier 5.x / 6.x Compatibility

[Mark Beacom]

Copier recently released the new major 6.x which (as of 6.0) modifies the jinja2 templating patterns (notably from brackets to braces).
This causes incompatibility issues if _envops configuration isn't explicitly set.
Per the copier documentation, backwards compatibility with 5.x can be achieved by explicitly setting the block_, comment_, and variable_ values for start and end string configuration values.

ref: https://copier.readthedocs.io/en/stable/configuring/#envops