remove workaround for python issue

that was solved upstream: https://bugs.python.org/issue14983

alot/helper.py

@@ -607,19 +607,6 @@ def email_as_string(mail):
     g = Generator(fp, mangle_from_=False, maxheaderlen=78)
     g.flatten(mail)
     as_string = RFC3156_canonicalize(fp.getvalue())
-
-    if isinstance(mail, MIMEMultipart):
-        # Get the boundary for later
-        boundary = mail.get_boundary()
-
-        # Workaround for http://bugs.python.org/issue14983:
-        # Insert a newline before the outer mail boundary so that other mail
-        # clients can verify the signature when sending an email which contains
-        # attachments.
-        as_string = re.sub(r'--(\r\n)--' + boundary,
-                           r'--\g<1>\g<1>--' + boundary,
-                           as_string, flags=re.MULTILINE)
-
     return as_string