DEPR: Deprecate is_copy (#18801)
Not all tests for this branch on the fork are passing, but these are the same tests that fail for the clean master and appear to be unrelated to the changes.

References to is_copy in tests were changed to the internal attribute _is_copy assuming that, though is_copy is not meant to be a public attribute, checking the state of this attribute is necessary to check consistent internal behavior.

Closes #18801.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18812?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@b2a02bd`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `95.83%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18812/graphs/tree.svg?width=650&src=pr&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/18812?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #18812   +/-   ##
=========================================
  Coverage          ?   91.62%           
=========================================
  Files             ?      154           
  Lines             ?    51407           
  Branches          ?        0           
=========================================
  Hits              ?    47103           
  Misses            ?     4304           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.49% <95.83%> (?)` | |
| #single | `40.84% <41.66%> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18812?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/accessors.py](https://codecov.io/gh/pandas-dev/pandas/pull/18812/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9hY2Nlc3NvcnMucHk=) | `89.36% <100%> (ø)` | |
| [pandas/core/indexing.py](https://codecov.io/gh/pandas-dev/pandas/pull/18812/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhpbmcucHk=) | `92.81% <100%> (ø)` | |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/18812/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `95.92% <95.23%> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18812?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18812?src=pr&el=footer). Last update [b2a02bd...099cd33](https://codecov.io/gh/pandas-dev/pandas/pull/18812?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Hello @cbertinato! Thanks for updating the PR.















Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on December 19, 2017 at 12:20 Hours UTC Shall I squash these commits into one and resubmit, or can we do it on the merge? no need to squash

will be auto done on merge  I am by no means in a rush, but just wanted to check whether this is waiting on anything else. I believe that the requested changes have been made. @cbertinato : There's nothing for you to do ATM, but we're currently having build issues on Travis for our `master` branch.  @jreback is trying to resolve those first before we merge anything else. Roger that! thanks @cbertinato  When tab-completing in iPython, the ``FutureWarning`` for ``is_copy`` pops up.

I believe that ``'is_copy'`` should be added to ``_deprecations`` yep that is right - PR? PR #18922 