Django-qr-code

* Drop support for Django 2.1.
* Ensure compatibility with Django 3.1.

* Ensure compatibility with Django 3.0.
* Upgrade Pillow requirement to 7.1.
* Drop support for Python 3.5.
* Drop support for Django <2.2.
* More modern build environment and configuration for ReadTheDocs.

* Ensure compatibility with Django 2.1.
* Ensure compatibility with Django 2.2.
* Upgrade qr_code library from 5.3 to 6.1 (several fixes).
* Drop support for Python 3.4.
* Fixed error when generating qr code from lazy text. (1)
* Add support for customizing usage of URL signature token via template tags (allows to generate URLs for serving QR code images that do not include a signature token). (4)
* The caching of QR codes images could allow to bypass checking the user when external access is active.
* Upgrade Pillow requirement to 6.2.0 (CVE-2019-16865).
* Adopt a dedicated logger, and move message "Pillow is not installed. No support available for PNG format." from info to debug. (6)

* BREAKING CHANGES:
* QR code options have been factorized and now use the `QRCodeOptions` class.
* The context for rendering a QR code encoding a Wi-Fi configuration uses the dedicated `WifiConfig` class.
* The context for rendering a QR code encoding a contact detail uses the dedicated `ContactDetail` class.
* `qr_for_contact` and `qr_url_for_contact` keyword arg has been renamed from `contact_dict` to `contact_detail`.
* `qr_for_wifi` and `qr_url_for_wifi` keyword arg has been renamed from `wifi_dict` to `wifi_config`.
* Reorganize code and split qr_code.py into several modules.

The changes mentioned above might break the compatibility with code using qr_code.py's API directly, but template tags are not impacted, except for `qr_for_contact`, `qr_url_for_contact`, `qr_for_wifi`, and `qr_url_for_wifi` if they were using a keyword argument.
* Other changes:
* Added support for `error_correction` parameter when generating a QR code.
* Added support for `coordinates` keyword argument to `qr_for_geolocation`, `qr_for_google_maps`, `qr_url_for_geolocation`, and `qr_url_for_google_maps`.
* Additions to documentation.
* Added ability to use a `QRCodeOptions` instance with `options` keyword argument in all tags.
* Bug fixes:
* Fixed non-closed <img> tag when generating embedded PNG image.
* Escape colon char (':') if it appears within a contact detail or a wifi configuration.
* Add a second terminal semi-colon at the end of the text representing a wifi configuration, as recommended in some sources.

* Fixed unescaped chars when generating QR code for a contact.
* Simplify handling of default values for QR code options.
* Add documentation about what a QR code can encode.

* Added support for multiple new tags:
* `qr_for_email` and `qr_url_for_email`
* `qr_for_tel` and `qr_url_for_tel`
* `qr_for_sms` and `qr_url_for_sms`
* `qr_for_geolocation` and `qr_url_for_geolocation`
* `qr_for_google_maps` and `qr_url_for_google_maps`
* `qr_for_youtube` and `qr_url_for_youtube`
* `qr_for_google_play` and `qr_url_for_google_play`
* `qr_for_contact` and `qr_url_for_contact`
* `qr_for_wifi` and `qr_url_for_wifi`
* Reformat documentation on the demo site for better readability.
* Drop support for Django <1.11.