diff --git a/mercurial/pycompat.py b/mercurial/pycompat.py
--- a/mercurial/pycompat.py
+++ b/mercurial/pycompat.py
@@ -154,7 +154,7 @@
     bytechr = struct.Struct(r'>B').pack
     byterepr = b'%r'.__mod__
 
-    class bytestr(bytes):
+    class _bytestr(bytes):
         """A bytes which mostly acts as a Python 2 str
 
         >>> bytestr(), bytestr(bytearray(b'foo')), bytestr(u'ascii'), bytestr(1)
@@ -208,7 +208,7 @@
         """
 
         def __new__(cls, s=b''):
-            if isinstance(s, bytestr):
+            if isinstance(s, _bytestr):
                 return s
             if not isinstance(
                 s, (bytes, bytearray)
@@ -398,7 +398,7 @@
     unicode = unicode
     bytechr = chr
     byterepr = repr
-    bytestr = str
+    _bytestr = str
     iterbytestr = iter
     maybebytestr = identity
     sysbytes = identity
@@ -504,3 +504,8 @@
     return tempfile.NamedTemporaryFile(
         mode, bufsize, suffix=suffix, prefix=prefix, dir=dir, delete=delete
     )
+
+
+def bytestr(s=b''):
+    # type: (Union[bytes,str]) -> _bytestr
+    return _bytestr(s)
diff --git a/mercurial/urllibcompat.py b/mercurial/urllibcompat.py
--- a/mercurial/urllibcompat.py
+++ b/mercurial/urllibcompat.py
@@ -105,7 +105,7 @@
     def quote(s, safe=r'/'):
         # bytestr has an __iter__ that emits characters. quote_from_bytes()
         # does an iteration and expects ints. We coerce to bytes to appease it.
-        if isinstance(s, pycompat.bytestr):
+        if isinstance(s, pycompat._bytestr):
             s = bytes(s)
         s = urllib.parse.quote_from_bytes(s, safe=safe)
         return s.encode('ascii', 'strict')