When using a modern curl we can download download multiple urls
in parallel which avoids connection setup overhead and is generally
more efficient. Use when it's detected.
TODO: ensure both old and new curl are tested automatically via
the testsuite.
Only py3.7+ has ThreadingHTTPServer and SimpleHTTPRequestHandler
that can take a directory argument. We could reimplement this
on py36 (easy for threading, harder for missing directory) but
instead this commit just skips tests that try to use a
ThreadingHTTPServer.
Remove once we no longer support py3.6.
To test the curl sources it is very useful to have a small httpd
server that can serve an arbitrary directory. This helper will
ensure that via:
```python
with with osbuild.testutil.net.http_serve_directory(fake_httpd_root) as httpd:
port = httpd.server_port
# download from http://localhost:{port}/<any-path-under-httpd-root>
```
