Use psycopg[binary] on all platforms (#262)

Since:
- the binary wheel package is much faster to install than the compiled
  from source sdist, which is beneficial in an interactive tutorial
  scenario
- the platform conditionals result in a more complicated requirements
  file (and confusing pip output about ignoring the `psycopg` package)
  which isn't ideal for beginners following the tutorial
- it results in more deterministic app/dependencies (since the libpq
  version is now bundled with the package rather than being dynamically
  linked), and whilst this is a trade-off, it's more in keeping with the
  modern app approach of pinning dependencies using lockfiles etc

GUS-W-19240129.

Author: edmorley

requirements.txt

@@ -3,10 +3,5 @@ gunicorn>=23,<24
 dj-database-url>=3,<4
 whitenoise>=6,<7
 
-# Uncomment these lines to use a Postgres database. Both are needed, since in production
-# (which uses Linux) we want to install from source, so that security updates from the
-# underlying Heroku stack image are picked up automatically, thanks to dynamic linking.
-# On other platforms/in development, the precompiled binary package is used instead, to
-# speed up installation and avoid errors from missing libraries/headers.
-#psycopg[c]; sys_platform == "linux"
-#psycopg[binary]; sys_platform != "linux"
+# Uncomment to use a Postgres database.
+#psycopg[binary]