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

CONTRIBUTORS

@@ -67,6 +67,7 @@ Mariusz Rusiniak
 Mark Hirota
 Matt Good
 Matt Jeffery
+Matthew Kenigsberg
 Mattieu Agopian
 Mehdi Abaakouk
 Michael Manganiello

docs/changelog/1656.bugfix.rst

@@ -0,0 +1 @@
+Allow escaping curly braces in setenv. - by :user:`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

tests/unit/config/test_config.py

@@ -2074,6 +2074,15 @@ def test_factors_in_setenv(self, newconfig):
         assert configs["py27"].setenv["X"] == "1"
         assert "X" not in configs["py36"].setenv
 
+    def test_curly_braces_in_setenv(self, newconfig):
+        inisource = """
+            [testenv]
+            setenv =
+                VAR = \{val\}
+        """
+        configs = newconfig([], inisource).envconfigs
+        assert configs["python"].setenv["VAR"] == "{val}"
+
     def test_factor_use_not_checked(self, newconfig):
         inisource = """
             [tox]