Add a testutil extra-require and add gitpython to it

Closes #5486

ChangeLog

@@ -43,6 +43,12 @@ Release date: TBA
 
   Closes #5322
 
+* Added a ``testutil`` extra require to the packaging, as ``gitpython`` should not be a dependency
+  all the time but is still required to use the primer helper code in ``pylint.testutil``. You can
+  install it with ``pip install pylint[testutil]``.
+
+  Closes #5486
+
 * Added several checkers to deal with unicode security issues
   (see `Trojan Sources <https://trojansource.codes/>`_ and
   `PEP 672 <https://www.python.org/dev/peps/pep-0672/>`_ for details) that also

requirements_test_min.txt

@@ -1,6 +1,5 @@
--e .
+-e .[testutil]
 # astroid dependency is also defined in setup.cfg
 astroid==2.9.3  # Pinned to a specific version for tests
 pytest~=7.0
 pytest-benchmark~=3.4
-gitpython>3

setup.cfg

@@ -56,6 +56,9 @@ install_requires =
     typing-extensions>=3.10.0;python_version<"3.10"
 python_requires = >=3.6.2
 
+[options.extras_require]
+testutil=gitpython>3
+
 [options.packages.find]
 include =
     pylint*