[#3350] upgrade pip in hammer for rhel

Fixes this on RHEL 8: You are using pip version 9.0.3, however version 24.0 is available.

Author: andrei-pavel

hammer.py

@@ -1763,6 +1763,8 @@ def link_pg_config():
             # kea-packaging points Kea to the venv using --with-sphinx.
             execute('python3 -m venv ~/venv',
                     env=env, timeout=60, check_times=check_times)
+            execute('~/venv/bin/pip install --upgrade pip',
+                    env=env, timeout=120, check_times=check_times)
             execute('~/venv/bin/pip install sphinx sphinx-rtd-theme',
                     env=env, timeout=120, check_times=check_times)