(deps): add testing dependencies: django, pytest, etc

- had to pin to python^3.5 as it's the common denominator between deps
  - add .python-version file to use python3.7 w/ pyenv for this project
  - will have to include python^2.7 again in order to publish a
    universal wheel compatible with 2.7 as the poetry python dep is
    used for both deps and publishing
  - will also have to add dep versions against python 2.7 in order to
    test against it and lower Django versions
    - either in Poetry itself or with something like Tox?
    - gitignore poetry.lock bc multiple pythons and dep versions

Author: agilgur5

.gitignore

@@ -1,3 +1,6 @@
+# we use multiple versions of Python and deps, so lockfile doesn't quite fit
+# may add it back in later once Poetry usage stabilizes
+poetry.lock
 
 # Created by https://www.gitignore.io/api/python
 

.python-version

@@ -0,0 +1 @@
+3.7.4

pyproject.toml

@@ -64,9 +64,13 @@ keywords=[
 "Tracker" = "https://github.com/agilgur5/django-serializable-model/issues"
 
 [tool.poetry.dependencies]
-python = "^2.7 || ^3"
+python = "^3.5"
 
 [tool.poetry.dev-dependencies]
+django = "^2.2"
+pytest = "^5.1"
+pytest-django = "^3.5"
+pytest-cov = "^2.7"
 
 [build-system]
 requires = ["poetry>=0.12"]