Change default smart connection to ssh on macOS and remove paramiko from requirements.txt (#54738)

* Remove default use of paramiko connection plugin on macOS
    This fix was originally to work around a bug that caused a kernel panic on macOS
    that has since been fixed.
* Remove paramiko from requirements.txt
* Move paramiko checking to common place
* Drop the warnings obfiscation code
* Update pip installation instructions to reflect upstream instructions
* Fix tests on CentOS 6 (Python 2.6) that now show Python deprecation warnings
* Add changelog fragment

test/integration/targets/assert/runme.sh

@@ -25,6 +25,9 @@ run_test() {
    sed -i -e 's/ *$//' "${OUTFILE}.${testname}.stdout"
    sed -i -e 's/ *$//' "${OUTFILE}.${testname}.stderr"
 
+   # Scrub deprication warning that shows up in Python 2.6 on CentOS 6
+   sed -i -e '/RandomPool_DeprecationWarning/d' "${OUTFILE}.${testname}.stderr"
+
    diff -u "${ORIGFILE}.${testname}.stdout" "${OUTFILE}.${testname}.stdout" || diff_failure
    diff -u "${ORIGFILE}.${testname}.stderr" "${OUTFILE}.${testname}.stderr" || diff_failure
 }