syntax: adjust the magic strings for the changes since file-5.10

Since file-5.10 (end of 2011), libmagic identifies a C file in most
cases as "C source" instead of as "C program".  Nano's magic strings
for some other files didn't match any more what file-5.32 currently
produces, either.  So, they have been adjusted, new ones added, and
old ones deleted.

This fixes https://savannah.gnu.org/bugs/?52445.

syntax/python.nanorc

@@ -2,6 +2,7 @@
 
 syntax "python" "\.py$"
 header "^#!.*python[-0-9._]*"
+magic "Python script"
 linter pyflakes
 comment "#"