Issue #23511: Port email-simple.py to Python 3.

Also, update email examples to use the context manager version of open().

Patch by Baptiste Mispelon.

Doc/includes/email-headers.py

 # Import the email modules we'll need
 from email.parser import Parser
 
-#  If the e-mail headers are in a file, uncomment this line:
-#headers = Parser().parse(open(messagefile, 'r'))
+# If the e-mail headers are in a file, uncomment these two lines:
+# with open(messagefile) as fp:
+#     headers = Parser().parse(fp)
 
 #  Or for parsing headers in a string, use:
 headers = Parser().parsestr('From: <user@example.com>\n'

Doc/includes/email-mime.py

@@ -20,9 +20,8 @@ msg.preamble = 'Our family reunion'
 for file in pngfiles:
     # Open the files in binary mode.  Let the MIMEImage class automatically
     # guess the specific image type.
-    fp = open(file, 'rb')
+    with open(file, 'rb') as fp:
         img = MIMEImage(fp.read())
-    fp.close()
     msg.attach(img)
 
 # Send the email via our own SMTP server.

Doc/includes/email-read-alternative-new-api.py

@@ -12,7 +12,8 @@ from email.parser import BytesParser
 from imaginary import magic_html_parser
 
 # In a real program you'd get the filename from the arguments.
-msg = BytesParser(policy=policy.default).parse(open('outgoing.msg', 'rb'))
+with open('outgoing.msg', 'rb') as fp:
+    msg = BytesParser(policy=policy.default).parse(fp)
 
 # Now the header items can be accessed as a dictionary, and any non-ASCII will
 # be converted to unicode:

Doc/includes/email-simple.py

@@ -6,10 +6,9 @@ from email.mime.text import MIMEText
 
 # Open a plain text file for reading.  For this example, assume that
 # the text file contains only ASCII characters.
-fp = open(textfile, 'rb')
-# Create a text/plain message
-msg = MIMEText(fp.read())
-fp.close()
+with open(textfile) as fp:
+    # Create a text/plain message
+    msg = MIMEText(fp.read())
 
 # me == the sender's email address
 # you == the recipient's email address