NANOGrav 9-yr GWB data tutorial notebook incompatible with Python 3

[nithyanandan]

See also issue #227

My operating system name and version and system architecture:

$ lsb_release -a
LSB Version:	:core-4.1-amd64:core-4.1-noarch:cxx-4.1-amd64:cxx-4.1-noarch:desktop-4.1-amd64:desktop-4.1-noarch:languages-4.1-amd64:languages-4.1-noarch:printing-4.1-amd64:printing-4.1-noarch
Distributor ID:	CentOS
Description:	CentOS Linux release 7.8.2003 (Core)
Release:	7.8.2003
Codename:	Core

$ hostnamectl
...
Operating System: CentOS Linux 7 (Core)
CPE OS Name: cpe:/o:centos:centos:7
Kernel: Linux 3.10.0-1127.10.1.el7.x86_64
Architecture: x86-64

Running Python 3.7

In using nanograv/enterprise git # f66648f, I found that it is incompatible with Python 3 due to the handling of string literals and print functions.

Also, there is a missing closing single quote in enclosing the text file name containing the sampled chain causing a SyntaxError.

[nithyanandan]

I've made the Python 3 fixes and can issue a pull request, but perhaps after addressing issue #227 first?