Remove Python 3.8+ restriction, as only Python 3.8+ is supported.

test_requirements.txt

@@ -1,10 +1,10 @@
-appdirs==1.4.4; python_version >= '3.8'
+appdirs==1.4.4
 apprise==1.3.0
-attrs==22.2.0; python_version >= '3.8'
-black==23.3.0; python_version >= '3.8'
+attrs==22.2.0
+black==23.3.0
 certifi==2023.7.22
 chardet==5.1.0
-click==8.1.3; python_version >= '3.8'
+click==8.1.3
 codespell==2.2.4
 colorama==0.4.6
 coverage==7.2.3