Let "make" install build and twine if in a virtual environment

If a virtual environment (created by venv or virtualenv) is active,
running "make release" or "make force_release" now automatically
installs/upgrades the "build" and "twine" packages in it. This is
only done if "make" is run in a virtual environment.

This can be a fresh environment: neither the project nor its
dependencies need to be installed in it. Because the "build" module
is not currently used in any tests and running "make" in a virtual
environment takes care of installing "build" (and "twine"), "build"
is now removed from test-requirements.txt.

The publishing instructions in the readme are updated accordingly,
to mention the optional step of creating and activating a virtual
environment, and to briefly clarify why one might want to do that.

Running "make" outside a virtual environment remains supported,
except that, due to recent changes, whatever environment it is run
in needs to have a usable "build" module.

Makefile

@@ -15,6 +15,8 @@ release: clean
 	make force_release
 
 force_release: clean
+	# IF we're in a virtual environment, add build tools
+	test -z "$$VIRTUAL_ENV" || pip install -U build twine
 	python3 -m build --sdist --wheel
 	twine upload dist/*
 	git push --tags origin main

README.md

@@ -188,13 +188,15 @@ Please have a look at the [contributions file][contributing].
 
 ### How to make a new release
 
-- Update/verify the **version** in the `VERSION` file
-- Update/verify that the `doc/source/changes.rst` changelog file was updated
-- Commit everything
-- Run `git tag -s <version>` to tag the version in Git
-- Run `make release`
+- Update/verify the **version** in the `VERSION` file.
+- Update/verify that the `doc/source/changes.rst` changelog file was updated.
+- Commit everything.
+- Run `git tag -s <version>` to tag the version in Git.
+- _Optionally_ create and activate a [virtual environment](https://packaging.python.org/en/latest/guides/installing-using-pip-and-virtual-environments/#creating-a-virtual-environment) using `venv` or `virtualenv`.\
+(When run in a virtual environment, the next step will automatically take care of installing `build` and `twine` in it.)
+- Run `make release`.
 - Close the milestone mentioned in the _changelog_ and create a new one. _Do not reuse milestones by renaming them_.
-- Got to [GitHub Releases](https://github.com/gitpython-developers/GitPython/releases) and publish a new one with the recently pushed tag. Generate the changelog.
+- Go to [GitHub Releases](https://github.com/gitpython-developers/GitPython/releases) and publish a new one with the recently pushed tag. Generate the changelog.
 
 ### How to verify a release (DEPRECATED)
 

test-requirements.txt

@@ -1,5 +1,4 @@
 black
-build
 coverage[toml]
 ddt>=1.1.1, !=1.4.3
 mypy