Migrate Booktype to Python 3

[Copilot]

Migrates codebase from Python 2 to Python 3.6+. Updates Django from 1.11 to 2.2 LTS and modernizes dependencies.

Dependency Updates

• Python 3.6+ with Django 2.2 LTS (minimum Python 3 compatible Django)
• Updated dependencies: djangorestframework 3.11+, celery 4.4+, redis 3.0+, Pillow 6.0+
• Replaced django-rest-swagger → drf-yasg, BeautifulSoup → beautifulsoup4
• Removed simplejson (native json module)

Django 2.0+ Compatibility

• All __unicode__() → __str__() in models (18 classes)
• Added on_delete=models.CASCADE to ForeignKey fields (59 fields)

Python 3 Syntax

• Imports: urllib2 → urllib.request, urlparse → urllib.parse, StringIO → io.StringIO
• Exception handling: except E, e: → except E as e:
• Dict methods: .iteritems() → .items(), .has_key(x) → x in dict
• Type checks: unicode() → str(), basestring → str
• Print statements → print functions
• Renamed async variables (reserved keyword in Python 3.5+)
• Fixed lambda tuple unpacking: lambda (a, b): → lambda item: ... item[0] ... item[1]

String/Bytes Handling

# Before (Python 2)
f = open('file.txt', 'r')
content = str(f.read(), 'utf-8')

# After (Python 3)
with open('file.txt', 'r', encoding='utf-8') as f:
    content = f.read()

All 387 Python files compile successfully with Python 3.12.

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