Use a "test" extra instead of tests_require

Because tests_require is deprecated since setuptools 41.5.0 with
the intention that it will be removed in some future version (noted
in https://setuptools.pypa.io/en/latest/references/keywords.html).

It is somewhat unintuitive for GitPython to have a "test" extra, as
it makes it so "GitPython[test]" can be specified for installation
from PyPI to get test dependencies, even though the PyPI package
doesn't include the unit test themselves.

However, this makes the statement in README.md that the installer
takes care of both requirements.txt and test-requirements.txt
dependencies fully true, instead of moving further away from that.

Because it is now possible to make an editable GitPython install
with test as well as minimal dependencies installed, this commit
also updates the readme to document and recommend this.

Author: EliahKagan

README.md

@@ -89,9 +89,11 @@ gh repo clone GitPython
 Having cloned the repo, create and activate your [virtual environment](https://docs.python.org/3/tutorial/venv.html). Then make an [editable install](https://pip.pypa.io/en/stable/topics/local-project-installs/#editable-installs):
 
 ```bash
-pip install -e .
+pip install -e ".[test]"
 ```
 
+In the less common case that you do not want to install test dependencies, `pip install -e .` can be used instead.
+
 ### Limitations
 
 #### Leakage of System Resources
@@ -120,20 +122,49 @@ On _Windows_, make sure you have `git-daemon` in your PATH. For MINGW-git, the `
 exists in `Git\mingw64\libexec\git-core\`; CYGWIN has no daemon, but should get along fine
 with MINGW's.
 
-Ensure testing libraries are installed.
-In the root directory, run: `pip install -r test-requirements.txt`
+#### Install test dependencies
+
+Ensure testing libraries are installed. This is taken care of already if you installed with:
+
+```bash
+pip install -e ".[test]"
+```
+
+Otherwise, you can run:
+
+```bash
+pip install -r test-requirements.txt
+```
+
+#### Test commands
+
+To test, run:
 
-To lint, run: `pre-commit run --all-files`
+```bash
+pytest
+```
+
+To lint, run:
 
-To typecheck, run: `mypy -p git`
+```bash
+pre-commit run --all-files
+```
 
-To test, run: `pytest`
+To typecheck, run:
 
-For automatic code formatting run: `black git`
+```bash
+mypy -p git
+```
+
+For automatic code formatting, run:
+
+```bash
+black git
+```
 
-Configuration for flake8 is in the ./.flake8 file.
+Configuration for flake8 is in the `./.flake8` file.
 
-Configurations for mypy, pytest and coverage.py are in ./pyproject.toml.
+Configurations for `mypy`, `pytest`, `coverage.py`, and `black` are in `./pyproject.toml`.
 
 The same linting and testing will also be performed against different supported python versions
 upon submitting a pull request (or on each push if you have a fork with a "main" branch and actions enabled).

setup.py

@@ -95,7 +95,7 @@ def build_py_modules(basedir: str, excludes: Sequence = ()) -> Sequence:
     package_dir={"git": "git"},
     python_requires=">=3.7",
     install_requires=requirements,
-    tests_require=requirements + test_requirements,
+    extras_require={"test": test_requirements},
     zip_safe=False,
     long_description=long_description,
     long_description_content_type="text/markdown",