fix: include cached-property as a conditional dependency

Before this commit, we were loading cached-property as a dependency in
`setup.py`, which is flawed, as this choice happens when the package is built
and not when the package is installed. I.e. whether osc-tiny depends on
cached-property depends on the builder's python version, not on the user's.

Author: dcermak

.github/workflows/default.yml

@@ -25,7 +25,7 @@ jobs:
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip
-          pip install -r requirements.txt -r requirements_pre38.txt responses
+          pip install -r requirements.txt responses
       - name: Test
         run: python -m unittest discover
   test:

requirements.txt

@@ -4,3 +4,4 @@ python-dateutil
 pytz
 pyyaml
 urllib3
+cached-property; python_version < '3.8'

requirements_pre38.txt

@@ -1,6 +1,5 @@
 #!/usr/bin/env python
 # -*- coding: utf-8 -*-
-from sys import version_info
 from setuptools import setup, find_packages
 
 
@@ -13,10 +12,6 @@ def _filter(requires):
     with open("requirements.txt", "r") as fh:
         requirements += _filter(fh.readlines())
 
-    if version_info.minor < 8:
-        with open("requirements_pre38.txt", "r") as fh:
-            requirements += _filter(fh.readlines())
-
     return requirements