update pip

.evergreen/run-mod-wsgi-tests.sh

@@ -21,6 +21,7 @@ PYTHON_VERSION=$(${PYTHON_BINARY} -c "import sys; sys.stdout.write('.'.join(str(
 # Ensure the C extensions are installed.
 ${PYTHON_BINARY} -m venv --system-site-packages .venv
 source .venv/bin/activate
+pip install -U pip
 python -m pip install -e .
 
 export MOD_WSGI_SO=/opt/python/mod_wsgi/python_version/$PYTHON_VERSION/mod_wsgi_version/$MOD_WSGI_VERSION/mod_wsgi.so