tests: target Python 3.8 as the minimum Python version

This version is already two releases out of support, but is "only" 7
years old so is probably still being used in the real world. Document
this version along with some other testing dependencies.  Remove code
support for earlier versions. Disable ruff checks that need a newer
version.

docs/INTERNALS.md

@@ -64,6 +64,19 @@ these or later versions:
 - perl           5.8 (2002-07-19), on Windows: 5.22 (2015-06-01)
 - Visual Studio  2010 10.0 (2010-04-12 - 2020-07-14)
 
+## Testing
+
+Certain tests require the following packages to operate. In some cases, tests
+that do not find the necessary requirements are automatically skipped.
+
+- OpenSSL        (see above)
+- nghttp2        (see above)
+- OpenSSH
+- perl           (see above)
+- pytest
+- Python         3.8 (2019-10-14)
+- stunnel
+
 ## Library Symbols
 
 All symbols used internally in libcurl must use a `Curl_` prefix if they are