split mypy requirements to its own file

requirements/requirements-check.txt

@@ -1,13 +1,6 @@
 # requirements for code style; linters, code analysis and formatting tools
 invoke==2.2.0
 black==24.2.0
-mypy==1.9.0
 isort==5.13.2
 ruff==0.3.2
-
-# The following are for mypy
-# some version of pytest required for checking types. Perhaps any recent
-# version of pytest will do.
-pytest
-types-colorama
-time-machine==2.14.0
+-r requirements-mypy.txt

requirements/requirements-mypy.txt

@@ -0,0 +1,15 @@
+# mypy 1.9.0 only supports Python 3.8+
+mypy==1.9.0; python_version>='3.8'
+# Last mypy version to support Python 3.7
+mypy==1.4.1; python_version=='3.7'
+
+
+# The following installs are for mypy.
+pytest -c requirements-test.txt
+
+types-colorama
+types-colorama==0.4.15.12; python_version>='3.7'
+
+time-machine==2.14.0; python_version>='3.8'
+# Last time-machine version to support Python 3.7
+time-machine==2.10.0; python_version>='3.8'