Use older mypy for py 37, 38, 39

Makefile

@@ -25,9 +25,9 @@ LINT_FILE_IGNORES = "$(LIB_NAME)/__init__.py:F401,F403 \
  $(LIB_NAME)/sampling/__init__.py:F401 \
  $(LIB_NAME)/utils/__init__.py:F401"
 
-# Python 3.7 uses a separate test requirements file
-ifeq ("$(VERSION_PYTHON)", "3.7")
- TEST_REQUIREMENTS = "tests_requirements_37.txt"
+# Older Python versions use a separate test requirements file
+ifeq ($(filter $(VERSION_PYTHON),3.7 3.8 3.9),$(VERSION_PYTHON))
+ TEST_REQUIREMENTS = "tests_requirements_pyold.txt"
 else
  TEST_REQUIREMENTS = "tests_requirements.txt"
 endif

tests_requirements_37.txt → tests_requirements_pyold.txt

@@ -1,12 +1,12 @@
-# Test requirements specific to Python 3.7
+# Test requirements specific to Python 3.7, 3.8 and 3.9.
 
 # Code quality tools:
 black==21.7b0
 codecov
 click==8.0.4
 flake8==4.0.1
 isort==5.10.1
-mypy<1.4.0 # Some newer versions of mypy cause issues with Python 3.7.
+mypy<1.4.0 # Newer mypy versions cause issues with older Python ('Self' from 'typing_extensions').
 pytest
 pytest-cov
 pytest-random-order