astropy/ccdproc: 2.1.1

There are several minor fixes in this release:

• Improve integration of ImageFileCollection with image combination and document that integration [#762]

• Add memory_profiler as a test requirement [#739]
• Updated test suite to use absolute, not relative imports [#735]

• test_image_collection.py in the test suite no longer produces permanent files on disk and cleans up after itself. [#738]
• Change Combiner to allow accepting either a list or a generator [#757]
• ImageFileCollection now correctly returns an empty collection when an existing collection is filtered restrictively enough to remove all files. [#750]
• Logging now preserves all of the arguments when the keyword argument names are not used. [#756]

These folks made code contributions to this release, which wouldn't have happened without them:

• Larry Bradley (@larrybradley)
• Timothy P. Ellsworth-Bowers (@tbowers7)
• Yash Gondhalekar (@Yash-10)
• Kelvin Lee (@laserkelvin)
• Pey Lian Lim (@pllim)
• Jenna Ryon (@jryon)

Thanks to these people who have helped by opening multiple issues with ccdproc -- many of the improvements come from these issues:

• Juan Cabanela (@JuanCab)
• @mheida
• Sara Ogaz (@SaOgaz)
• Jean-Paul Ventura (@jvntra)
• Kerry Paterson (@KerryPaterson)
• Jane Rigby (@janerigby)
• Kris Stern (@kakirastern)
• Alexa Villaume (@AlexaVillaume)
• Brian York (@york-stsci)
• Sylvielsstfr (@sylvielsstfr)