build/pkgs/snappy: Update dependencies, requirements.txt, add comments

Author: Matthias Koeppe

build/pkgs/snappy/dependencies

@@ -1,4 +1,10 @@
-$(PYTHON) | $(PYTHON_TOOLCHAIN)
+$(PYTHON) decorator future ipython cypari | $(PYTHON_TOOLCHAIN) sagelib
 
 ----------
 All lines of this file are ignored except the first.
+
+The dependency cypari above is actually cypari2.
+
+An installed sagelib is needed when snappy is installed from source (instead of a wheel)
+because snappy's setup.py tests its presence to adjust dependencies.
+https://trac.sagemath.org/ticket/31180

build/pkgs/snappy/requirements.txt

@@ -1,3 +1,8 @@
-# The --no-deps flag is just there to prevent it from pulling in cypari (!= cypari2).
-# In fact, just sage -pip install snappy does the trick but installs the (unused even by SnapPy in this context) cypari module.
---no-deps snappy spherogram plink FXrays snappy_manifolds
+# Note: As of 2021-01, snappy will pull in cypari (!= cypari2) as a dependency
+# if installed as a wheel but will actually use Sage's cypari2.
+# cypari contains a statically linked copy of pari and other libraries
+# and will remain completely unused (wastes 30M).  Snappy is about 165M.
+# See https://trac.sagemath.org/ticket/31180
+snappy
+# An optional database (110M uncompressed)
+snappy_15_knots