Suggest using pyenv for manging Python versions

Pip fails to install some libraries listed in requirements.txt with
Python versions later than 3.11. Address this issue in the readme file
and suggest pyenv as a solution. Add pyenv's .python-version file to
gitignore.

Author: Gunnar Pruß

.gitignore

@@ -1,5 +1,6 @@
 .idea
 .vscode
+.python-version
 venv
 .venv
 docs/build

README.rst

@@ -25,6 +25,12 @@ Set up virtual environment
     python3 -m venv venv
     source ./venv/bin/activate
 
+Note: Some libraries included in requirements.txt only work in Python versions up to
+and including 3.11.  So if you run into troubles installing the required dependencies
+(see below), you might want to consider using `pyenv <https://github.com/pyenv/pyenv>`
+to install an additional version of Python on your system *before* creating the virtual
+environment.
+
 Install Requirements
 --------------------