Guard Lark.__doc__ assignment for tools.standalone

The debug flag is already saved in the cached file, but is not
resumed on loading the dumped file. To solve that,
 - add argument `debug` to parser_fronteds.WithLexer.deserialize
 - add argument `debug` lalr_parser.LALR_Parser.deserialize
 - propagate the value of the `debug` option on resuming a cached
   grammar, in lark.Lark._load

Python2.7 uses protocol 1 by default, which is not compatible with
class defining __slots__. On the other hand, all the Python versions
support versions >= 2.

The debug flag is already saved in the cached file, but is not
resumed on loading the dumped file. To solve that,
 - add argument `debug` to parser_fronteds.WithLexer.deserialize
 - add argument `debug` lalr_parser.LALR_Parser.deserialize
 - propagate the value of the `debug` option on resuming a cached
   grammar, in lark.Lark._load

Python2.7 uses protocol 1 by default, which is not compatible with
class defining __slots__. On the other hand, all the Python versions
support versions >= 2.

Run tests against Python 3.9

updated lark.lark with two small fixes

lark.lark didn't parse grammars that didn't have a trailing newline in the file, but lark itself has no problem parsing those grammars
this change makes lark.lark more accurate in reflecting what grammars are parsed by lark.

using `SIGNED_INT` instead of `INT` more closely matches the the grammar used by lark in `load_grammar.py`

Test print argument to lark.tools.standalone.main

Add print parameter to lark.tools.standalone.main