Trac #31180: Add snappy as a pip package

Following the instructions from #31176.

(see also https://snappy.math.uic.edu/installing.html#sagemath)

URL: https://trac.sagemath.org/31180
Reported by: mkoeppe
Ticket author(s): Matthias Koeppe
Reviewer(s): Nathan Dunfield

Author: Release Manager

build/pkgs/snappy/SPKG.rst

@@ -0,0 +1,18 @@
+snappy: Topology and geometry of 3-manifolds, with a focus on hyperbolic structures
+===================================================================================
+
+Description
+-----------
+
+Studying the topology and geometry of 3-manifolds, with a focus on hyperbolic structures.
+
+License
+-------
+
+GPLv2+
+
+Upstream Contact
+----------------
+
+https://pypi.org/project/snappy/
+

build/pkgs/snappy/dependencies

@@ -0,0 +1,10 @@
+$(PYTHON) decorator 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

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+# 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
+# cypari 2.4.0 has a broken sdist, https://trac.sagemath.org/ticket/31180
+cypari !=2.4.0
+# An optional database (110M uncompressed)
+snappy_15_knots

build/pkgs/snappy/type

@@ -0,0 +1 @@
+optional