Drop support for Python 3.5 and 3.6, add support for 3.8 and 3.9

Python 3.5 is EOL already, and Blender never used version 3.6 (it went
straight to 3.7), hence 3.6 was never tested explicitly.

Python 3.7 or newer is required from now on.

CHANGELOG.md

@@ -3,6 +3,11 @@
 This file logs the changes that are actually interesting to users (new features,
 changed functionality, fixed bugs).
 
+## Version 1.5 (in development)
+
+- Drop support for Python 3.5 and 3.6, and add support for 3.8 and 3.9.
+
+
 ## Version 1.4.1 (2021-07-22)
 
 - Reverted unintended bump of required Python version.

pyproject.toml

@@ -21,7 +21,7 @@ classifiers = [
 s3 = ["boto3"]
 
 [tool.poetry.dependencies]
-python = "^3.5"
+python = "^3.7"
 requests = "^2.11"
 
 # For S3 storage support:

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 isolated_build = true
-envlist = py35, py37
+envlist = py37, py38, py39
 
 [testenv]
 whitelist_externals = poetry