Add "make lint" to lint without auto-formatting

As on CI and with tox (but not having to build and create and use a
tox environment).

This may not be the best way to do it, since currently the
project's makefiles are otherwise used only for things directly
related to building and publishing. However, this seemed like a
readily available way to run the moderately complex command that
produces the same behavior as on CI or with tox, but outside a tox
environment. It may be possible to improve on this in the future.

Makefile

@@ -1,8 +1,11 @@
-.PHONY: all clean release force_release
+.PHONY: all lint clean release force_release
 
 all:
 	@awk -F: '/^[[:alpha:]].*:/ && !/^all:/ {print $$1}' Makefile
 
+lint:
+	SKIP=black-format pre-commit run --all-files --hook-stage manual
+
 clean:
 	rm -rf build/ dist/ .eggs/ .tox/
 

README.md

@@ -148,11 +148,8 @@ To lint, and apply automatic code formatting, run:
 pre-commit run --all-files
 ```
 
-Code formatting can also be done by itself by running:
-
-```
-black .
-```
+- Linting without modifying code can be done with: `make lint`
+- Auto-formatting without other lint checks can be done with: `black .`
 
 To typecheck, run: