Linkchecker

Changes:
- Minimum supported version of Beautiful Soup is 4.8.1

Fixes:
- wsgi: Fix failure due to status logging being enabled

Features:
- Uses Python 3
- C extension modules have been replaced, now uses Beautiful Soup
- Documentation converted to reStructuredText and generated with Sphinx

Changes:
- cmdline: Remove options replaced by plugins and made ineffective in 9.0
- configuration: Update proxy settings support for GNOME 3 and KDE 5
- configuration: login entries must now match the case of form element names
- logging: blacklist has been renamed to failures
- checking: Handle HTTP status code 429: Too Many Requests with
a new warning: WARN_URL_RATE_LIMITED, instead of an error
- checking: Use timeout when fetching login forms and robots.txt
- checking: login forms with only one field are supported
- checking: slack and whatsapp added to the list of ignored schemes
- tests: Test coverage has been increased
- biplist is no longer used because plistlib now supports binary files
- dnspython and miniboa are no longer included
- Custom MANIFEST check replaced with check-manifest
- Code now passes flake8 checks

Fixes:
- configuration: status=0 is no longer ignored
- logging: Fix CSV logger not recognising base part setting
- logging: Fix CSV output containing increasing number of null byte characters.
- checking: Fix treating data: URIs in srcset values as links
- checking: Fix critical exception if srcset value ends with a comma
- checking: Fix critical exception when parsing a URL with a ]
- plugins: The AnchorCheck plugin is working again
- plugins: The W3C validation API has changed, CssSyntaxCheck has been updated,
HtmlSyntaxCheck plugin is disabled
- doc: Multiple man page and other documentation updates

Features:
- checking: Support itms-services: URLs.
Closes: GH bug 532
- checking: Support XDG Base Directory Specification for configuration
and data.
Closes: GH bug 44
- add Dockerfile
- use xdg dirs for config & data
- use tox for tests and fix travis build
- add --no-robots commandline flag
- Added plugin for parsing and checking links in Markdown files

Changes:
- installation: Remove dependency on msgfmt.py by pre-generating the
*.mo files and adding them to version control.
Reason was the difficulty to run msgfmt.py under both Python 2 and 3.
- checking: When checking SSL certificates under POSIX systems try
to use the system certificate store.
- logging: improved debugging by also enabling urllib3 output
- remove third-party packages and use them as dependency
- Allow wayback-format urls without affecting atom 'feed' urls
- Move dev requirements into dev-requirements.txt
- Crawl HTML attributes in deterministic order
- Remove platform-specific installer stuff and ensure a build .whl wheel file can be built.
- Move GUI files to separate project

Fixes:
- checking: Correct typos in the proxy handling code.
Closes: GH bug 536
- checking: Add to default HTTP client headers instead of replacing.
- cmdline: Reactivate paging of help pages.
- requirements: Fix requests module version check.
Closes: GH bug 548
- load cookies from the --cookiefile correctly
- fix incorrect call to the logging module
- Fix TypeError: hasattr(): attribute name must be string
- fix HTTPS URL checks

This release is the first release after the fork. It aims to factor in critical bugs that were present for a while like issues with HTTPS and requests. This synchronizes the source code with the Debian package and is the first release of the stable 9.3.x branch. New feature development and other non-critical changes happen on the master branch.

This is not yet released on PyPI and on the website, see issues 4 and 5 for this.