asciidocapi: Fix crash on load of the loader

[aerostitch]

Hi guys!

As describe by Simon in Debian bug #945391, when trying to instantiate the asciidocapi with the version I put in Debian based on commit 51d7c14, the loader fails:

$ python3
Python 3.7.5 (default, Oct 27 2019, 15:43:29) 
[GCC 9.2.1 20191022] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import io
>>> import sys
>>> sys.path.append('/usr/share/asciidoc')
>>> import asciidocapi
>>> infile = io.StringIO('Test')
>>> outfile = io.StringIO()
>>> adoc = asciidocapi.AsciiDocAPI()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/share/asciidoc/asciidocapi.py", line 209, in __init__
    self.__import_asciidoc()
  File "/usr/share/asciidoc/asciidocapi.py", line 244, in __import_asciidoc
    module = importlib.util.module_from_spec(spec)
  File "<frozen importlib._bootstrap>", line 580, in module_from_spec
AttributeError: 'NoneType' object has no attribute 'loader'
>>> 

This patch was proposed by Simon Ruderich in his bug report and it works!

$ python3
Python 3.7.5 (default, Oct 27 2019, 15:43:29) 
[GCC 9.2.1 20191022] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import io
>>> import sys
>>> sys.path.append('/usr/share/asciidoc')
>>> import asciidocapi
>>> infile = io.StringIO('Test')
>>> outfile = io.StringIO()
>>> adoc = asciidocapi.AsciiDocAPI()
>>> adoc.execute(infile, outfile)
>>> outfile.getvalue()
*** content stripped here because it's a big html code but it works! ***

Thanks for considering this.

Joseph

[MasterOdin]

Can reproduce the bug by just renaming asciidoc.py to asciidoc in the repo and then running the code, as I guess spec_from_file_location requires a file extension to exist for it to work.

[aerostitch]

Thx! :)