Add .python-version instead of runtime.txt

Author: Ivaylo-Bachvarov

.gitignore

@@ -81,9 +81,6 @@ target/
 profile_default/
 ipython_config.py
 
-# pyenv
-.python-version
-
 # pipenv
 #   According to pypa/pipenv#598, it is recommended to include Pipfile.lock in version control.
 #   However, in case of collaboration, if having platform-specific dependencies or dependencies

.python-version

@@ -0,0 +1 @@
+3.13

README.md

@@ -341,7 +341,7 @@ To get an overview of how Heroku deployment works, we recommend reading this fir
 1. `Procfile`
    - Comes with default `web`, `worker` and `beat` processes.
    - Additionally, there's a `release` phase to run migrations safely, before releasing the new build.
-1. `runtime.txt`
+1. `.python-version`
    - Simply specifies the Python version to be used.
 1. `requirements.txt`
    - Heroku requires a root-level `requirements.txt`, so we've added that.