Allow example to work with or without ape-tokens installed

[fubuloubu]

silverback/example.py

Line 7 in bfba34b

from ape_tokens import tokens # type: ignore[import]

do something like:

try:
    # You can use any Ape's extensive array of plugins and tools
    from ape_tokens import tokens
except ImportError:
    # NOTE: Must install tokenlists to use this feature
    tokens = {}

...

YFI = tokens.get("YFI", Contract("<YFI address on mainnet>"))
...

[linear]

SBK-602 Allow exampe to work with or without `ape-tokens` installed

[fubuloubu]

Acceptance criteria:

uv tool install silverback
wget https://raw.githubusercontent.com/ApeWorX/silverback/refs/heads/main/example.py
silverback run example --network ethereum:mainnet