Fix root when frozen with pyinstaller

[johnnymetz]

When freezing a WeasyPrint application with PyInstaller, root needs to be set to sys._MEIPASS, which is the absolute path to the bundle folder (source:
https://pythonhosted.org/PyInstaller/runtime-information.html). _MEIPASS appears to be uniquely set by PyInstaller so my code change simply checks for this attribute and sets the appropriate root.

You can recreate the issue by performing the following steps.
Environment:
macOS 10.12.6
python 3.6 (or 3.4) via anaconda

1. Create a basic weasyprint program weasyprint_frozen.py:

import weasyprint
print(weasyprint.__version__)
input('Press enter to exit the program: ')

2. Freeze the program with PyInstaller. Note some files from WeasyPrint need to be bundled with the frozen application (that's what the --add-data options are for):

pip install pyinstaller
pyinstaller weasyprint_frozen.py --name 'Weasyprint App' --log-level ERROR --clean --add-data '/Users/Jonnymetz/anaconda/lib/python3.6/site-packages/pyphen/dictionaries:pyphen/dictionaries' --add-data '/Users/Jonnymetz/anaconda/lib/python3.6/site-packages/weasyprint/css/html5_ph.css:css' --add-data '/Users/Jonnymetz/anaconda/lib/python3.6/site-packages/weasyprint/css/html5_ua.css:css' --onefile

As WeasyPrints currently stands, the frozen application will not run and the traceback should look similar to this:

Traceback (most recent call last):
  File "weasyprint_frozen.py", line 1, in <module>
  File "<frozen importlib._bootstrap>", line 971, in _find_and_load
  File "<frozen importlib._bootstrap>", line 955, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 665, in _load_unlocked
  File "/Users/Jonnymetz/anaconda/envs/pyweasy36/lib/python3.6/site-packages/PyInstaller/loader/pyimod03_importers.py", line 631, in exec_module
    exec(bytecode, module.__dict__)
  File "site-packages/weasyprint/__init__.py", line 376, in <module>
  File "<frozen importlib._bootstrap>", line 971, in _find_and_load
  File "<frozen importlib._bootstrap>", line 955, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 665, in _load_unlocked
  File "/Users/Jonnymetz/anaconda/envs/pyweasy36/lib/python3.6/site-packages/PyInstaller/loader/pyimod03_importers.py", line 631, in exec_module
    exec(bytecode, module.__dict__)
  File "site-packages/weasyprint/html.py", line 40, in <module>
  File "site-packages/weasyprint/__init__.py", line 261, in __init__
  File "contextlib.py", line 81, in __enter__
  File "site-packages/weasyprint/__init__.py", line 338, in _select_source
FileNotFoundError: [Errno 2] No such file or directory: '/Users/Jonnymetz/github/basic_flask/dist/css/html5_ua.css'
[24858] Failed to execute script weasyprint_frozen

With my code change, the frozen application should run as expected:

0.41
Press enter to exit the program: 

[liZe]

Thank you for this pull request.

Well… I'm not really happy each time I add some code that can't be tested and that's specific to some random external tools. The "frozen" hack had been added a long time ago in #269, and I'm really sad to see that there's no way to use the same standard hack with two different tools that are supposed to do the same thing. I don't know what I'll do when someone else will come saying "please add some random code using some random non-standard private variable for my XXX tool that's supposed to work with standard Python programs but that actually needs to change the libraries in a way that will probably change in two years or so when YYY will replace XXX".

OK. That's not your fault. I won't cry this time for two little lines that will help you and may help others.

[Cheaterman]

PyInstaller isn't a random tool. I had issues packaging WeasyPrint but I didn't get to follow this in detail (the --add-data args), I will try again now. Thanks to both of you!