Fix versions in requirements.txt for python 3.5 and 3.9 (#20507)

Packages listed in current requirements.txt file cannot be installed in
python 3.5 and 3.9:
* Pillow doesn't have a candidate for 3.9, it was already removed
  in #20407, but added again by mistake in #16883.
* zipp package needed by pytest works with different versions depending
  on the version of python, version that works with python 3.5 doesn't work
  with other versions.

libbeat/tests/system/requirements.txt

@@ -25,7 +25,6 @@ more-itertools==8.4.0
 ordered-set==3.1.1
 packaging==20.4
 parameterized==0.7.0
-Pillow==7.2.0
 pluggy==0.13.1
 py==1.9.0
 pycodestyle==2.4.0
@@ -45,4 +44,4 @@ toml==0.10.1
 urllib3==1.24.2
 wcwidth==0.2.5
 websocket-client==0.47.0
-zipp==3.1.0
+zipp>=1.2.0,<=3.1.0

script/check_python_requirements.sh

@@ -0,0 +1,46 @@
+#!/bin/bash
+#
+# Helper script to check that packages defined in a requirements.txt
+# file can be installed in different Python versions, it checks by
+# default the requirements.txt file for libbeat tests.
+#
+#    Usage: check_python_requirements.sh /path/to/requirements.txt
+#
+# VERSIONS environment variable can be set to a space-separated list
+# of versions of python to test with.
+#
+
+set -e
+
+function abspath() {
+	local path=$1
+	if [ -d "$path" ]; then
+		cd "$path"; pwd; cd - > /dev/null
+	else
+		echo $(abspath "$(dirname "$path")")/$(basename "$path")
+	fi
+}
+
+BEATS_PATH=$(abspath "$(dirname "${BASH_SOURCE[0]}")"/..)
+
+VERSIONS=${VERSIONS:-3.5 3.6 3.7 3.8 3.9-rc}
+REQUIREMENTS=${1:-${BEATS_PATH}/libbeat/tests/system/requirements.txt}
+
+if [ ! -f "$REQUIREMENTS" ]; then
+	echo "Requirements file doesn't exist: $REQUIREMENTS"
+	exit -1
+fi
+
+REQUIREMENTS=$(abspath "$REQUIREMENTS")
+
+echo "Versions: $VERSIONS"
+echo "Requirements file: $REQUIREMENTS"
+
+for version in $VERSIONS; do
+	echo "==== Version: $version"
+
+	docker run -it --rm -v "$REQUIREMENTS":/requirements.txt python:$version \
+		python -m pip install -q -r /requirements.txt
+
+	echo "==== OK"
+done