Allow escaping curly braces in setenv

Currently braces can be escaped, but the backslashes are not removed,
making it impossible to add a plain curly brace

Author: mkenigs

src/tox/config/__init__.py

@@ -382,7 +382,9 @@ def get(self, name, default=None):
                 return os.environ.get(name, default)
             self._lookupstack.append(name)
             try:
-                self.resolved[name] = res = self.reader._replace(val)
+                res = self.reader._replace(val)
+                res = res.replace("\\{", "{").replace("\\}", "}")
+                self.resolved[name] = res
             finally:
                 self._lookupstack.pop()
             return res