tweaks: remove some ineffectual parts from header-line regexes

Adding "[abc]*" does not restrict the recognized header line in any way.

Also, improve the header-line regex for shell scripts, because it should
not match "barunscript" (for example).

syntax/python.nanorc

@@ -1,7 +1,7 @@
 ## Here is an example for Python.
 
 syntax python "\.py$"
-header "^#!.*python[-0-9._]*"
+header "^#!.*python"
 magic "Python script"
 linter pyflakes
 comment "#"