After all these years, go back to requirements.txt

Now that `pip` is sometimes able to install packages — if wheels are
available, and the Python version matches the era to which the packages
are pinned — let’s try a switch back to a `requirements.txt`.  Even
though it only works with Python 3.6.  (Python 3.7, which my Ubuntu
laptop features as its default, dies during a `pip install` while trying
to compile AstroPy.)  This might pave the way for easy interoperation
with tools like Binder in the future.

.python-version

@@ -0,0 +1 @@
+3.6.8

requirements.sh

@@ -1,27 +0,0 @@
-#!/bin/bash
-
-set -e
-
-if conda --version >/dev/null 2>&1
-then
-    conda install \
-          astropy=3.0.1 \
-          beautifulsoup4=4.6.0 \
-          html5lib=1.0.1 \
-          lxml=4.2.1 \
-          mock=2.0.0 \
-          numpy=1.14.2 \
-          pandas=0.23.3 \
-          python=3.6 \
-          pytz \
-          sphinx=1.7.2 \
-
-else
-    if ! python --version 2>&1 | grep -q 2.6
-    then pip install astropy
-    fi
-    pip install mock numpy sphinx pytz
-fi
-pip install de405==1997.1 de423==2010.1
-pip install https://github.com/brandon-rhodes/assay/archive/master.zip
-pip install -e .

requirements.txt

@@ -0,0 +1,11 @@
+astropy==3.2.2
+beautifulsoup4==4.6.0
+html5lib==1.0.1
+lxml==4.4.1
+mock==2.0.0
+numpy==1.14.2
+pandas==0.23.3
+pytz
+sphinx==1.7.2
+https://github.com/brandon-rhodes/assay/archive/master.zip
+-e .