Clarify currently supported Python versions

The Python 3.8 build currently fails on travis, this should be dealt with in a future PR along with updating the travis config and pyproject.toml

.travis.yml

@@ -1,6 +1,8 @@
 dist: xenial   # required for Python >= 3.7
 language: python
-python: "3.7"
+python:
+  - 3.6
+  - 3.7
 git:
   depth: false
 before_install:

poetry.lock

@@ -385,7 +385,7 @@ pytz = "*"
 
 [metadata]
 content-hash = "9896cf59c7552b6ad95219ee5555c7445a3fab39c2e4f4c6f3d991a36635e44b"
-python-versions = "^3.7"
+python-versions = ">=3.6.0, <3.8.0"
 
 [metadata.hashes]
 appdirs = ["9e5896d1372858f8dd3344faf4e5014d21849c756c8d5701f78f8a103b372d92", "d8b24664561d0d34ddfaec54636d502d7cea6e29c3eaf68f3df6180863e2166e"]

pyproject.toml

@@ -13,7 +13,7 @@ homepage = "https://jrnl.sh"
 repository = "https://github.com/jrnl-org/jrnl"
 
 [tool.poetry.dependencies]
-python = "^3.7"
+python = ">=3.6.0, <3.8.0"
 pyxdg = "^0.26.0"
 cryptography = "^2.7"
 passlib = "^1.7"