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
#require symlink
$ origdir=`pwd`
$ hg init repo
$ cd repo
$ ln -s nothing dangling
avoid tar warnings about old timestamp
$ hg ci -d '2000-01-01 00:00:00 +0000' -qAm 'add symlink'
$ hg archive -t files ../archive
$ hg archive -t tar -p tar ../archive.tar
$ hg archive -t zip -p zip ../archive.zip
files
$ cd "$origdir"
$ cd archive
$ readlink.py dangling
dangling -> nothing
tar
$ cd "$origdir"
$ tar xf archive.tar
$ cd tar
$ readlink.py dangling
dangling -> nothing
#if unziplinks
zip
$ cd "$origdir"
$ unzip archive.zip > /dev/null 2>&1
$ cd zip
$ readlink.py dangling
dangling -> nothing
#endif
$ cd ..