Revert "python3: now points to python38"

This is going to require more work.

This reverts commit 2dc4ab36772dfaf124af4d90e99c68038f46baa0.

doc/languages-frameworks/python.section.md

···
 Versions 2.7, 3.5, 3.6, 3.7 and 3.8 of the CPython interpreter are available as
 respectively `python27`, `python35`, `python36`, `python37` and `python38`. The aliases
 `python2` and `python3` correspond to respectively `python27` and
-`python38`. The default interpreter, `python`, maps to `python2`. The PyPy
+`python37`. The default interpreter, `python`, maps to `python2`. The PyPy
 interpreters compatible with Python 2.7 and 3 are available as `pypy27` and
 `pypy3`, with aliases `pypy2` mapping to `pypy27` and `pypy` mapping to
 `pypy2`. The Nix expressions for the interpreters can be

pkgs/top-level/all-packages.nix

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   # When switching these sets, please update docs at ../../doc/languages-frameworks/python.md
   python = python2;
   python2 = python27;
-  python3 = python38;
+  python3 = python37;
   pypy = pypy2;
   pypy2 = pypy27;
   pypy3 = pypy36;