tests/readlink.py

hgdemandimport: disable on Python 3.5

The demand importer functionality isn't working at all on Python 3.5.
I'm not sure what's wrong.

Since it isn't working, let's disable it completely.

```
$ HGRCPATH= hyperfine -w 1 -r 50 -- "~/.pyenv/versions/3.5.9/bin/python ./hg version" \
  "HGDEMANDIMPORT=disable ~/.pyenv/versions/3.5.9/bin/python ./hg version"
Benchmark #1: ~/.pyenv/versions/3.5.9/bin/python ./hg version
  Time (mean ± σ):     163.7 ms ±   2.2 ms    [User: 148.5 ms, System: 15.7 ms]
  Range (min … max):   161.0 ms … 170.2 ms    50 runs

Benchmark #2: HGDEMANDIMPORT=disable ~/.pyenv/versions/3.5.9/bin/python ./hg version
  Time (mean ± σ):     164.3 ms ±   1.4 ms    [User: 148.2 ms, System: 16.6 ms]
  Range (min … max):   161.4 ms … 169.8 ms    50 runs
```

Differential Revision: https://phab.mercurial-scm.org/D7953

#!/usr/bin/env python

from __future__ import absolute_import, print_function

import errno
import os
import sys

for f in sys.argv[1:]:
    try:
        print(f, '->', os.readlink(f))
    except OSError as err:
        if err.errno != errno.EINVAL:
            raise
        print(f, '->', f, 'not a symlink')

sys.exit(0)