MNT: temporarily trim all .txt file extensions

Author: neutrinoceros

COPYING.txt COPYINGCOPYING.txt renamed to COPYING

@@ -56,6 +56,12 @@ include yt/utilities/lib/platform_dep_math.hpp
 include yt/utilities/lib/tsearch.c
 include yt/utilities/lib/tsearch.h
 
+include COPYING
+include doc/docstring_idioms
+include doc/source/analyzing/_static/axes_calculator_setup
+include yt/utilities/grid_data_format/docs/IRATE_notes
+include yt/utilities/grid_data_format/docs/gdf_specification
+
 include doc/README doc/activate doc/activate.csh doc/cheatsheet.tex
 exclude doc/cheatsheet.pdf
 include doc/extensions/README doc/Makefile
@@ -69,6 +75,7 @@ include yt/py.typed
 include yt/default.mplstyle
 
 prune yt/frontends/_skeleton
+
 recursive-include yt/frontends/amrvac *.par
 recursive-exclude requirements *.txt
 exclude minimal_requirements.txt