lib2to3's token.py and tokenize.py were initially copies of the respective
files from the standard library.  They were copied to allow Python 3 to read
Python 2's grammar.

Since 2006, lib2to3 grew to be widely used as a Concrete Syntax Tree, also for
parsing Python 3 code.  Additions to support Python 3 grammar were added but
sadly, the main token.py and tokenize.py diverged.

This change brings them back together, minimizing the differences to the bare
minimum that is in fact required by lib2to3.  Before this change, almost every
line in lib2to3/pgen2/tokenize.py was different from tokenize.py.  After this
change, the diff between the two files is only 200 lines long and is entirely
filled with relevant Python 2 compatibility bits.

Merging the implementations, there's numerous fixes to the lib2to3 tokenizer:

+ docstrings made as similar as possible
+ ported `TokenInfo`
+ ported `tokenize.tokenize()` and `tokenize.open()`
+ removed Python 2-only implementation cruft
+ made Unicode identifier handling the same
+ made string prefix handling the same
+ added Ellipsis to the Special group
+ Untokenizer backported bugfixes:
  - 5e6db31
  - 9dc3a36
  - 5b8d2c3
  - e411b66
  - BPO-2495
+ `detect_encoding` tries to figure out a filename and `find_cookie` uses
  the filename in error messages, if available
+ `find_cookie` bugfix: BPO-14990
+ BPO-16152: tokenizer doesn't crash on missing newline at the end of the
  stream (added \Z (end of string) to PseudoExtras)

Improvements to token.py:

+ taken from the current Lib/token.py
+ tokens renumbered to match Lib/token.py
+ `__all__` properly defined
+ ASYNC, AWAIT and BACKQUOTE exist under different numbers (100 + old number)
+ ELLIPSIS added
+ ENCODING added