[3.7] bpo-35128: Fix spacing issues in warning.warn() messages. (GH-10268). (GH-10280)

(cherry picked from commit 4b5e62dbb22a3593e0db266c12f805b727a42b00)

Co-authored-by: Pablo Aguiar <scorphus@gmail.com>
diff --git a/Lib/poplib.py b/Lib/poplib.py
index d8a62c0..9796f0d 100644
--- a/Lib/poplib.py
+++ b/Lib/poplib.py
@@ -436,7 +436,7 @@
                                  "exclusive")
             if keyfile is not None or certfile is not None:
                 import warnings
-                warnings.warn("keyfile and certfile are deprecated, use a"
+                warnings.warn("keyfile and certfile are deprecated, use a "
                               "custom context instead", DeprecationWarning, 2)
             self.keyfile = keyfile
             self.certfile = certfile