Explicitly specify python2 for ubuntu2004

 (#2420)

* Explicitly specify python2 for ubuntu2004

* add a note explaining python 2 usage

* explain why python version must be explicitly given

Co-authored-by: Oleg Pudeyev <[email protected]>

Author: p-mongo

.evergreen/run-tests.sh

@@ -1,5 +1,17 @@
 #!/bin/bash
 
+# Note that mlaunch is executed with (and therefore installed with) Python 2.
+# The reason for this is that in the past, some of the distros we tested on
+# had an ancient version of Python 3 that was unusable (e.g. it couldn't
+# install anything from PyPI due to outdated TLS/SSL implementation).
+# It is likely that all of the current distros we use have a recent enough
+# and working Python 3 implementation, such that we could use Python 3 for
+# everything.
+#
+# Note that some distros (e.g. ubuntu2004) do not contain a `python' binary
+# at all, thus python2 or python3 must be explicitly specified depending on
+# the desired version.
+
 set -e
 set -o pipefail
 
@@ -274,7 +286,7 @@ if test -n "$OCSP_MOCK_PID"; then
   kill "$OCSP_MOCK_PID"
 fi
 
-python -m mtools.mlaunch.mlaunch stop --dir "$dbdir"
+python2 -m mtools.mlaunch.mlaunch stop --dir "$dbdir"
 
 if test -n "$FLE" && test "$DOCKER_PRELOAD" != 1; then
   # Terminate all kmip servers... and whatever else happens to be running