mention 3.8 requirement in contrib docs (#2856)

* mention 3.8 requirement in contrib docs

* add change.md for PR#2856

* docs: fix grammar and wording

changes/2856-paxcodes.md

@@ -0,0 +1 @@
+Update contrib docs re: python version to use for building docs.

docs/contributing.md

@@ -41,9 +41,10 @@ git clone git@github.com:<your username>/pydantic.git
 cd pydantic
 
 # 2. Set up a virtualenv for running tests
-virtualenv -p `which python3.7` env
+virtualenv -p `which python3.8` env
 source env/bin/activate
-# (or however you prefer to setup a python environment, 3.6 will work too)
+# Building docs requires 3.8. If you don't need to build docs you can use
+# whichever version; 3.6 will work too.
 
 # 3. Install pydantic, dependencies, test dependencies and doc dependencies
 make install