replace: treat single backslash replacement as literal

if the replaced value is a single backslash, then process replace on the
left and right parts of the value separately so that the replacement
cannot act as an escape in other parts of the value.

Fixes tox-dev#2732

src/tox/config/loader/ini/replace.py

@@ -43,6 +43,9 @@ def replace(conf: Config, loader: IniLoader, value: str, args: ConfigLoadArgs) -
             # want to enforce escaping curly braces, e.g. it should work to write: env_list = {py39,py38}-{,dep}
             end = end + 1
             continue
+        elif replaced == "\\":
+            # a replaced backslash should NOT affect subsequent iterations, so recurse on left and right
+            return replaced.join(replace(conf, loader, vp, args) for vp in [value[:start], value[end + 1 :]])
         new_value = f"{value[:start]}{replaced}{value[end + 1:]}"
         end = 0  # if we performed a replacement start over
         if new_value == value:  # if we're not making progress stop (circular reference?)