[doc] explain what .python-version does

test/functional/README.md

@@ -20,6 +20,10 @@ don't have test cases for.
 - Where possible, try to adhere to [PEP-8 guidelines](https://www.python.org/dev/peps/pep-0008/)
 - Use a python linter like flake8 before submitting PRs to catch common style
   nits (eg trailing whitespace, unused imports, etc)
+- The oldest supported Python version is specified in [doc/dependencies.md](/doc/dependencies.md).
+  Consider using [pyenv](https://github.com/pyenv/pyenv), which checks [.python-version](/.python-version),
+  to prevent accidentally introducing modern syntax from an unsupported Python version.
+  The Travis linter also checks this, but [possibly not in all cases](https://github.com/bitcoin/bitcoin/pull/14884#discussion_r239585126).
 - See [the python lint script](/test/lint/lint-python.sh) that checks for violations that
   could lead to bugs and issues in the test code.
 - Avoid wildcard imports where possible