Author: Kacper Kowalik <xarthisius.kk@gmail.com>
Description: Copy keys to a list, since orignal map is modified
Url: https://github.com/astropy/astropy/issues/4477
Bug:  https://github.com/astropy/astropy/issues/4460
--- a/astropy/wcs/wcs.py
+++ b/astropy/wcs/wcs.py
@@ -988,7 +988,7 @@
         """
         # Never pass SIP coefficients to wcslib
         # CTYPE must be passed with -SIP to wcslib
-        for key in (m.group() for m in map(SIP_KW.match, header.keys())
+        for key in (m.group() for m in map(SIP_KW.match, list(header))
                     if m is not None):
             del header[key]