Update dependencies and test 3.7 on xenial

[jarrodmillman]

Xenial is not officially supported yet.

See travis-ci/travis-ci#9815

[jarrodmillman]

Pygraphviz doesn't work on Python 3.7 yet: pygraphviz/pygraphviz#164

[dschult]

Removing the default_opener command allows the tests to finish.
All default_opener does on linux is run xdg-open, but that could be opening the file using a variety of programs depending on the xenial setup. I don't see any reason that adg-open would hang and cause the tests to not complete. I'll need some help tracking down how the configuration of xenial will allow xdg-open to finish and return control.

The osx test on 3.7 is still not working, but it looks like it just doesn't run the tests. The log says script.sh is started, but there is no output after that and the test is marked as failed.

[jarrodmillman]

I thought I already had a fix for xdg-open and pngs in

tools/travis/linux_install.sh

The relevant part is

  # needed for view_graphviz and default_opener
  DIR=~/.local/share/applications
  mkdir -p $DIR
  FILE=$DIR/png.desktop
  cat <<EOF >$FILE
[Desktop Entry]
Name=png
MimeType=image/png;
Exec=/usr/bin/file
Type=Application
Terminal=true
NoDisplay=true
EOF

  xdg-mime default png.desktop image/png

It looks like there is an issue with xdg-open on xenial. Here is someone talking about a similar problem: https://askubuntu.com/questions/779717/xdg-open-stopped-working-since-16-04-upgrade

I am going to just make xdg-open an alias for file.
I'll take a look and see what changed.