allow salt=None again

CHANGES.rst

@@ -5,6 +5,8 @@ Unreleased
 
 -   Mark top-level names as exported so type checking understands
     imports in user projects. :pr:`240`
+-   The ``salt`` argument to ``Serializer`` and ``Signer`` can be
+    ``None`` again. :issue:`237`
 
 
 Version 2.0.0

src/itsdangerous/serializer.py

@@ -89,7 +89,7 @@ class Serializer:
     def __init__(
         self,
         secret_key: _t_secret_key,
-        salt: _t_str_bytes = b"itsdangerous",
+        salt: _t_opt_str_bytes = b"itsdangerous",
         serializer: _t.Any = None,
         serializer_kwargs: _t_opt_kwargs = None,
         signer: _t.Optional[_t_signer] = None,
@@ -102,7 +102,12 @@ def __init__(
         #: This allows a key rotation system to keep a list of allowed
         #: keys and remove expired ones.
         self.secret_keys: _t.List[bytes] = _make_keys_list(secret_key)
-        self.salt: bytes = want_bytes(salt)
+
+        if salt is not None:
+            salt = want_bytes(salt)
+            # if salt is None then the signer's default is used
+
+        self.salt = salt
 
         if serializer is None:
             serializer = self.default_serializer

src/itsdangerous/signer.py

@@ -120,7 +120,7 @@ class Signer:
     def __init__(
         self,
         secret_key: _t_secret_key,
-        salt: _t_str_bytes = b"itsdangerous.Signer",
+        salt: _t_opt_str_bytes = b"itsdangerous.Signer",
         sep: _t_str_bytes = b".",
         key_derivation: _t.Optional[str] = None,
         digest_method: _t.Optional[_t.Any] = None,
@@ -141,7 +141,12 @@ def __init__(
                 " digits, and '-_=' must not be used."
             )
 
-        self.salt: bytes = want_bytes(salt)
+        if salt is not None:
+            salt = want_bytes(salt)
+        else:
+            salt = b"itsdangerous.Signer"
+
+        self.salt = salt
 
         if key_derivation is None:
             key_derivation = self.default_key_derivation