docs(): update tutorial links,

    * update tutorial links use new url https://learn.astroyp.org
    * add .python-version to .gitignore

Tested via jupyter notebok as referenced in README.md, no issues noted.

.gitignore

@@ -8,4 +8,5 @@ IPython-*
 *.pyc
 *.tex
 exec*.ipynb
+.python-version
 .DS_Store

tutorials/User-Defined-Model/User-Defined-Model.ipynb

@@ -16,7 +16,7 @@
     "    * Compound models\n",
     "    * Custom models\n",
     "\n",
-    "This tutorial assumes the student knows how to fit data using `astropy.modeling`. This topic is covered in the [Models-Quick-Fit tutorial](http://www.astropy.org/astropy-tutorials/rst-tutorials/Models-Quick-Fit.html).\n",
+    "This tutorial assumes the student knows how to fit data using `astropy.modeling`. This topic is covered in the [Models-Quick-Fit tutorial](https://learn.astropy.org/tutorials/Models-Quick-Fit.html).\n",
     "\n",
     "## Keywords\n",
     "modeling, FITS, astrostatistics, matplotlib, model fitting, error bars, scatter plots\n",
@@ -32,7 +32,7 @@
     "<div class=\"alert alert-info\">\n",
     "\n",
     "**Note:** This tutorial assumes you have already gone through \n",
-    "    [Modeling 1](http://www.astropy.org/astropy-tutorials/rst-tutorials/Models-Quick-Fit.html),\n",
+    "    [Modeling 1](https://learn.astropy.org/tutorials/Models-Quick-Fit.html),\n",
     "    which provides an introduction to `astropy.modeling`\n",
     "    \n",
     "\n",

tutorials/units-and-integration/units-and-integration.ipynb

@@ -24,7 +24,7 @@
     "\n",
     "## Companion Content\n",
     "* http://synphot.readthedocs.io/en/latest/\n",
-    "* [Using Astropy Quantities for astrophysical calculations](http://www.astropy.org/astropy-tutorials/rst-tutorials/quantities.html)\n",
+    "* [Using Astropy Quantities for astrophysical calculations](https://learn.astropy.org/tutorials/quantities.html)\n",
     "\n",
     "## Summary\n",
     "In this tutorial, we will use the examples of the Planck function and the stellar initial mass function (IMF) to illustrate how to integrate numerically, using the trapezoidal approximation and Gaussian quadrature. We will also explore making a custom class, an instance of which is callable in the same way as a function. In addition, we will encounter `astropy`'s built-in units, and get a first taste of how to convert between them. Finally, we will use $\\LaTeX$ to make our figure axis labels easy to read."