This was fixed in main by 4a85a9077136. Resolving.

this is a possible patch using the first approach

easy_install obscurely substitutes the builtin file type with a bound method.

setup.py imports version.py which imports util.py which fails subclassing file
because of that

the easiest fix without touching easy_install itself is to move the import of
util in version.py inside the function using it, an alternative would be to make
version not depend on util anymore by redefining nulldev there.

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