Fixed setup commands in contributing doc (#543)

* fixed description in contributing doc and made it more future-proof

* rst, not md

---------

Co-authored-by: Norman Rzepka <code@normanrz.com>

docs/contributing.rst

@@ -92,12 +92,12 @@ Creating a development environment
 To work with the NumCodecs source code, it is recommended to set up a Python virtual
 environment and install all NumCodecs dependencies using the same versions as are used by
 the core developers and continuous integration services. Assuming you have a Python
-3 interpreter already installed, and have also installed the virtualenv package, and
-you have cloned the NumCodecs source code and your current working directory is the root of
-the repository, you can do something like the following::
+3 interpreter already installed matching the ``requires-python`` constraint from
+``pyproject.toml``, and you have cloned the NumCodecs source code and your
+current working directory is the root of the repository, you can do something
+like the following::
 
-    $ mkdir -p ~/pyenv/numcodecs-dev
-    $ virtualenv --no-site-packages --python=/usr/bin/python3.9 ~/pyenv/numcodecs-dev
+    $ python3 -m venv ~/pyenv/numcodecs-dev
     $ source ~/pyenv/numcodecs-dev/bin/activate
     $ pip install -e .[docs,test,msgpack,zfpy]