bpo-38914 Do not require email field in setup.py. (GH-17388)

When checking `setup.py` and when the `author` field was provided, but
the `author_email` field was missing, erroneously a warning message was
displayed that the `author_email` field is required.

The specs do not require the `author_email`field:
https://packaging.python.org/specifications/core-metadata/#author

The same is valid for `maintainer` and `maintainer_email`.

The warning message has been adjusted.

modified:   Doc/distutils/examples.rst
modified:   Lib/distutils/command/check.py


https://bugs.python.org/issue38914

command/check.py

@@ -80,8 +80,11 @@ class check(Command):
     def check_metadata(self):
         """Ensures that all required elements of meta-data are supplied.
 
-        name, version, URL, (author and author_email) or
-        (maintainer and maintainer_email)).
+        Required fields:
+            name, version, URL
+
+        Recommended fields:
+            (author and author_email) or (maintainer and maintainer_email))
 
         Warns if any are missing.
         """
@@ -97,15 +100,15 @@ class check(Command):
         if metadata.author:
             if not metadata.author_email:
                 self.warn("missing meta-data: if 'author' supplied, " +
-                          "'author_email' must be supplied too")
+                          "'author_email' should be supplied too")
         elif metadata.maintainer:
             if not metadata.maintainer_email:
                 self.warn("missing meta-data: if 'maintainer' supplied, " +
-                          "'maintainer_email' must be supplied too")
+                          "'maintainer_email' should be supplied too")
         else:
             self.warn("missing meta-data: either (author and author_email) " +
                       "or (maintainer and maintainer_email) " +
-                      "must be supplied")
+                      "should be supplied")
 
     def check_restructuredtext(self):
         """Checks if the long string fields are reST-compliant."""