Upgrade pip for virtualenv

With this commit we upgrade pip to the latest version in the `make
install` target (used only for development). This avoids warnings about
an outdated version and let's us also use newer pip features in the
future.

Relates #707

Makefile

@@ -60,6 +60,7 @@ check-venv:
 	fi
 
 install: venv-create
+	. $(VENV_ACTIVATE_FILE); pip install --upgrade pip
 	. $(VENV_ACTIVATE_FILE); pip3 install -e .
 	# install tox for it tests
 	. $(VENV_ACTIVATE_FILE); pip3 install tox