Only make .tar.gz sdists when releasing

PEP 527, which was just accepted, says that there can only be one sdist
per release. Tarballs are already the more common sdist format on PyPI,
so let's go with that.

tools/toollib.py

@@ -1,56 +1,56 b''
 """Various utilities common to IPython release and maintenance tools.
 """
 from __future__ import print_function
 
 # Library imports
 import os
 
 # Useful shorthands
 pjoin = os.path.join
 cd = os.chdir
 
 # Constants
 
 # SSH root address of the archive site
 archive_user = '[e-mail]'
 archive_dir = 'archive.ipython.org'
 archive = '%s:%s' % (archive_user, archive_dir)
 
 # Build commands
 # Source dists
-sdists = './setup.py sdist --formats=gztar,zip'
+sdists = './setup.py sdist --formats=gztar'
 # Binary dists
 def buildwheels():
     sh('python3 setupegg.py bdist_wheel' % py)
 
 # Utility functions
 def sh(cmd):
     """Run system command in shell, raise SystemExit if it returns an error."""
     print("$", cmd)
     stat = os.system(cmd)
     #stat = 0  # Uncomment this and comment previous to run in debug mode
     if stat:
         raise SystemExit("Command %s failed with code: %s" % (cmd, stat))
 
 # Backwards compatibility
 c = sh
 
 def get_ipdir():
     """Get IPython directory from command line, or assume it's the one above."""
 
     # Initialize arguments and check location
     ipdir = pjoin(os.path.dirname(__file__), os.pardir)
 
     ipdir = os.path.abspath(ipdir)
 
     cd(ipdir)
     if not os.path.isdir('IPython') and os.path.isfile('setup.py'):
         raise SystemExit('Invalid ipython directory: %s' % ipdir)
     return ipdir
 
 try:
     execfile = execfile
 except NameError:
     def execfile(fname, globs, locs=None):
         locs = locs or globs
         exec(compile(open(fname).read(), fname, "exec"), globs, locs)