The present version of Mercurial does not appear to be easy_installable,
traceback attached. Curiously, things work, if I build an .egg out of it, and
easy_install the egg.

$ easy_install -U Mercurial
Searching for Mercurial
Reading http://pypi.python.org/simple/Mercurial/
Reading http://www.selenic.com/mercurial
Best match: mercurial 0.9.5
Downloading http://www.selenic.com/mercurial/release/mercurial-0.9.5.tar.gz
Processing mercurial-0.9.5.tar.gz
Running mercurial-0.9.5/setup.py -q bdist_egg --dist-dir
/tmp/easy_install-kRcJK_/mercurial-0.9.5/egg-dist-tmp-11dlHa
Traceback (most recent call last):
  File "/usr/bin/easy_install-2.5", line 8, in <module>
    load_entry_point('setuptools==0.6c7', 'console_scripts', 'easy_install-2.5')()
  File
"/usr/local/lib/python2.5/site-packages/setuptools-0.6c7-py2.5.egg/setuptools/command/easy_install.py",
line 1670, in main
  File
"/usr/local/lib/python2.5/site-packages/setuptools-0.6c7-py2.5.egg/setuptools/command/easy_install.py",
line 1659, in with_ei_usage
  File
"/usr/local/lib/python2.5/site-packages/setuptools-0.6c7-py2.5.egg/setuptools/command/easy_install.py",
line 1674, in <lambda>
  File "/usr/lib/python2.5/distutils/core.py", line 151, in setup
  File "/usr/lib/python2.5/distutils/dist.py", line 974, in run_commands
  File "/usr/lib/python2.5/distutils/dist.py", line 994, in run_command
  File
"/usr/local/lib/python2.5/site-packages/setuptools-0.6c7-py2.5.egg/setuptools/command/easy_install.py",
line 211, in run
  File
"/usr/local/lib/python2.5/site-packages/setuptools-0.6c7-py2.5.egg/setuptools/command/easy_install.py",
line 446, in easy_install
  File
"/usr/local/lib/python2.5/site-packages/setuptools-0.6c7-py2.5.egg/setuptools/command/easy_install.py",
line 471, in install_item
  File
"/usr/local/lib/python2.5/site-packages/setuptools-0.6c7-py2.5.egg/setuptools/command/easy_install.py",
line 655, in install_eggs
  File
"/usr/local/lib/python2.5/site-packages/setuptools-0.6c7-py2.5.egg/setuptools/command/easy_install.py",
line 930, in build_and_install
  File
"/usr/local/lib/python2.5/site-packages/setuptools-0.6c7-py2.5.egg/setuptools/command/easy_install.py",
line 919, in run_setup
  File
"/usr/local/lib/python2.5/site-packages/setuptools-0.6c7-py2.5.egg/setuptools/sandbox.py",
line 27, in run_setup
  File
"/usr/local/lib/python2.5/site-packages/setuptools-0.6c7-py2.5.egg/setuptools/sandbox.py",
line 63, in run
  File
"/usr/local/lib/python2.5/site-packages/setuptools-0.6c7-py2.5.egg/setuptools/sandbox.py",
line 29, in <lambda>
  File "setup.py", line 16, in <module>
  File "/tmp/easy_install-kRcJK_/mercurial-0.9.5/mercurial/version.py", line 16,
in <module>
  File "/tmp/easy_install-kRcJK_/mercurial-0.9.5/mercurial/util.py", line
1308,in <module>
TypeError: Error when calling the metaclass bases
    first argument must be callable