Update pip-tools

pip-tools <6.4.0 is incompatible with pip 21.3
See <jazzband/pip-tools#1503> for details.

Makefile

@@ -4,7 +4,7 @@ TEST_PROCESSES := $(if $(TEST_PROCESSES),$(TEST_PROCESSES),2)
 TEST_FILES := $(if $(TEST_FILES),$(TEST_FILES),tests/integration/test_*.py)
 MAX_LINE_LENGTH = $(shell cat .pylintrc | grep max-line-length | cut -d '=' -f 2 | xargs)
 
-PIP_TOOLS_VERSION = 6.2.0
+PIP_TOOLS_VERSION = 6.4.0
 PYTHON_PLATFORM := $(if $(PYTHON_PLATFORM),$(PYTHON_PLATFORM),manylinux2014_x86_64)
 PYTHON_VERSION := $(if $(PYTHON_VERSION),$(PYTHON_VERSION),36)