Bump Django to latest LTS release 1.11.20

This also adds back support for Python <= 3.7.

Related:
* https://gitlab.com/fdroid/fdroid-repomaker-flatpak/merge_requests/29
* https://gitlab.com/fdroid/repomaker/issues/197#note_114210540
* 931c60c521

README.md

@@ -49,13 +49,6 @@ On Debian, you can simply run this:
 
 `apt install python3-pip python3-wheel python3-dev virtualenv build-essential`
 
-Currently, Repomaker only works with Python versions minor than 3.7 due to
-[a bug in Django](https://github.com/django/django/commit/931c60c5216bd71bc11f489e00e063331cf21f40).
-This means that you need to install older versions of Python, e.g. version 3.6,
-if you are on a bleeding edge distro like Arch Linux.
-If you try to get this running on Arch Linux,
-[this comment](https://gitlab.com/fdroid/repomaker/issues/197#note_114210540) might help.
-
 #### Runtime
 
 * `keytool` from Java Runtime Environment (JRE)

requirements.txt

@@ -1,5 +1,5 @@
 -r debian/requirements.txt
-django<2.0
+django==1.11.20
 django-compressor
 libsass
 qrcode

setup.py

@@ -15,13 +15,13 @@ setup(
     description='Create F-Droid repositories with ease',
     license='AGPL-3.0',
     url='https://f-droid.org/repomaker/',
-    python_requires='>=3, <3.7',
+    python_requires='>=3',
     # List run-time dependencies here.  These will be installed by pip when
     # your project is installed. For an analysis of "install_requires" vs pip's
     # requirements files see:
     # https://packaging.python.org/en/latest/requirements.html
     install_requires=[
-        'django<2.0',
+        'django==1.11.20',
         'django-allauth',
         'django-tinymce',
         'django-js-reverse',