Move runtime.txt parsing into base class

[manics]

There's a discussion somewhere (I can't remember where) about using https://github.com/astrofrog/pypi-timemachine or similar so Python packages can be installed as if pip were run in the past, similar to R. If we do this we should define the cutoff date for Python in the same way as for R, which is to use runtime.txt.

Something to consider is we haven't formally defined what runtime.txt contains.

Our docs state it should be one of:

• name-version-yyyy-mm-dd: (r-<RVERSION>-<YYYY>-<MM>-<DD>)
• name-version: (python-x.y)
  

  repo2docker/docs/source/config_files.rst

  Lines 201 to 207 in e795060

  	Have ``python-x.y`` in ``runtime.txt`` to run the repository with Python version x.y.
  	See our `Python2 example repository <https://github.com/binder-examples/python2_runtime/blob/HEAD/runtime.txt>`_.
  	Have ``r-<RVERSION>-<YYYY>-<MM>-<DD>`` in ``runtime.txt`` to run the repository with R version RVERSION and libraries from a YYYY-MM-DD snapshot of `Posit Package Manager <https://packagemanager.posit.co/client/#/repos/2/overview>`__.
  	RVERSION can be set to 3.4, 3.5, 3.6, or to patch releases for the 3.5 and 3.6 series.
  	If you do not specify a version, the latest release will be used (currently R 3.6).
  	See our `R example repository <https://github.com/binder-examples/r/blob/HEAD/runtime.txt>`_.

Our tests say it can also be:

• name-yyyy-mm-dd
  

  repo2docker/tests/unit/test_r.py

  Line 38 in e795060

  ("r-2019-01-01", (2019, 1, 1)),

This PR handles all three formats. I've made a breaking change and redefined the existing .runtime property instead of creating a new one to avoid leaving an unused .runtime property that may cause confusion in the future.