Speed up import by replacing pkg_resources with importlib.resources

Speed up `import ligo.skymap` by up to a second by replacing uses of
`pkg_resources` with the new Python standard library module
`importlib.resources` (or, for Python < 3.7, the backport
`importlib_resources`). The old `pkg_resources` module is known to be
slow because it does a lot of work on startup.

See, for example,
[pypa/setuptools#926](pypa/setuptools#926) and
[pypa/setuptools#510](pypa/setuptools#510).

CHANGES.rst

@@ -5,7 +5,13 @@ Changelog
 0.2.1 (unreleased)
 ==================
 
-- No changes yet.
+- Speed up ``import ligo.skymap`` by up to a second by replacing uses of
+  ``pkg_resources`` with the new Python standard library module
+  ``importlib.resources`` (or, for Python < 3.7, the backport
+  ``importlib_resources``). The old ``pkg_resources`` module is known to be
+  slow because it does a lot of work on startup. (See, for example,
+  https://github.com/pypa/setuptools/issues/926 and
+  https://github.com/pypa/setuptools/issues/510.)
 
 0.2.0 (2020-04-21)
 ==================

ligo/skymap/plot/cmap.py

@@ -16,17 +16,22 @@
 #
 """Register some extra Matplotlib color maps"""
 
+try:
+    from importlib import resources
+except ImportError:
+    # FIXME: remove after dropping support for Python < 3.7
+    import importlib_resources as resources
+
 from matplotlib import cm
 from matplotlib import colors
 import numpy as np
-import pkg_resources
 
 __all__ = ()
 
 
 for name in ['cylon']:
     # Read in color map RGB data.
-    with pkg_resources.resource_stream(__name__, name + '.csv') as f:
+    with resources.open_text(__package__, name + '.csv') as f:
         data = np.loadtxt(f, delimiter=',')
 
     # Create color map.

setup.cfg

@@ -42,6 +42,7 @@ install_requires =
     astropy-healpix>=0.3  # https://github.com/astropy/astropy-healpix/pull/106
     healpy
     h5py
+    importlib-resources;python_version<'3.7'
     lalsuite>=6.53
     lscsoft-glue>=2.0.0
     ligo-gracedb>=2.0.1