(ci): add a Python 2.7 test

- needed to rm pyproject.toml because pip decided to read it during
  editable install and errored out
  - how it read the poetry section I don't even know or why that
    instead of setup.py I haven't a clue
  - also not sure why it didn't error out on the Python 3.4 test....
  - dafuq is going on here, it occurs even on job restarts...

Author: agilgur5

.travis.yml

@@ -4,6 +4,7 @@ python: '3.5'
 # test multiple Django versions
 matrix:
   include:
+    # Python 3.x envs
     - env: DJANGO_VERSION=2.2
     - env: DJANGO_VERSION=1.9
     - env: DJANGO_VERSION=1.5
@@ -15,6 +16,18 @@ matrix:
         - pip install django==1.5 pytest-django==2.9 pytest==3.5 pytest-cov==2.6
         - pip install -e .  # won't be able to do this if setup.py is removed!
 
+    # Python 2.7 envs
+    - env: DJANGO_VERSION=1.11  # latest Django that supports 2.7
+      python: '2.7'
+      # Poetry fails to resolve multi-constrant deps, so don't use it (https://github.com/sdispater/poetry/issues/666#issuecomment-531100022)
+      before_install: skip
+      install:
+        # pytest 5+ does not support Python 2.7
+        - pip install django==1.11 "pytest<5" pytest-django pytest-cov
+        - rm pyproject.toml  # editable install reads from here somehow and bugs out on 2.7!!??!!: https://travis-ci.org/agilgur5/django-serializable-model/jobs/584822609
+        - pip install -e .  # won't be able to do this if setup.py is removed!
+
+
 before_install: pip install poetry
 install:
   - poetry install