Rephrase Python version requirement and make it stand out more.

Summary: Resolves T2917

Reviewers: #reviewers, zack

Maniphest Tasks: T2917

Differential Revision: https://forge.softwareheritage.org/D4796

docs/developer-setup.rst

@@ -22,7 +22,7 @@ Install required dependencies
 -----------------------------
 
 Software Heritage requires some dependencies that are usually packaged by your
-package manager [1]. On Debian/Ubuntu-based distributions::
+package manager. On Debian/Ubuntu-based distributions::
 
   sudo wget https://www.postgresql.org/media/keys/ACCC4CF8.asc -O /etc/apt/trusted.gpg.d/postgresql.asc
   echo "deb http://apt.postgresql.org/pub/repos/apt/ $(lsb_release -cs)-pgdg main" | sudo tee -a /etc/apt/sources.list.d/pgdg.list
@@ -37,7 +37,7 @@ package manager [1]. On Debian/Ubuntu-based distributions::
       postgresql-autodoc graphviz plantuml inkscape \
       postgresql libpq-dev cassandra
 
-[1] python3.7 runtime version is the minimum required
+.. Note:: Python 3.7 or newer is required
 
 This installs basic system utilities, Python library dependencies, development tools,
 documentation tools and our main database management systems.