DEPR: GH10623 remove items from msgpack.encode for blocks
closes #10623 

hmm legacy file for 0.17.1 not passing.  I think it's tz-related.  Will check later.
 passing now.  had to make changes for `DatetimeTZBlock` (it wasn't tested?) and skip files packed in P2 with 0.17 :( due to #12142
 updated  
 fyi some of these fixes will prob address: https://github.com/pydata/pandas/issues/11455, maybe #11755 
 updated... but I forgot to squash :(  will do that later
 pls add a whatsnew as well (put in API changes).
make a note in `io.rst` that the writing format is changing in 0.18.0 so we won't have forward compat (IOW older version of pandas cannot read newer version)
 this would close #12142 as well? (should add a warning in the docs about this) (e.g. show your matrix)
 I can change all the keys to unicode, and then it will.  will update tonight
 Ah half way through but have to go.  I will get it done tonight but is that too late? :(
 sure no prob
 updated.  manually tested with python 2.7 and 3.5 on linux (zlib and no compress). 
 updated.  So this is how I tested P2 & P3 compatibility: use the script to generate legacy data for 0.18.0 in P2 and P3, put them in 0.18.0 folder, and let the tests pick them up.
 thanks!

I added some more test msgpacks for 0.17.1 to cover the bases
 https://travis-ci.org/pydata/pandas/jobs/109620001

I think we need to skip if blosc is not installed on these tests. 

can you do a PR for this?
