argparse module missing from dependencies / Python < 2.7

[Martin Borho]

After installation of u1db-python in a fresh virtualenv with Python 2.6.5 I got following error message after starting the server:

$ u1db-serve --verbose
Traceback (most recent call last):
  File "/home/martin/u1dbenv/bin/u1db-serve", line 49, in <module>
    sys.exit(main(sys.argv[1:]))
  File "/home/martin/u1dbenv/bin/u1db-serve", line 29, in main
    import argparse
ImportError: No module named argparse