Replace deprecated option

Deprecated: `experimental string processing` has been included in `preview` and deprecated. Use `preview` instead.

Replace deprecated option
Deprecated: `experimental string processing` has been included in `preview` and deprecated. Use `preview` instead.
496936f6 · Bo-Chun Chen · 62552999 · 496936f6
Commit 496936f6 authored 3 years ago by Bo-Chun Chen
--- a/pyproject.toml
+++ b/pyproject.toml
 [tool.black]
 line-length=79
 target-version=['py36']
-experimental_string_processing=true
+preview=true