ENH: return an OrderedDict from read_excel with sheetname=None/list #9930
Fix https://github.com/pydata/pandas/issues/9930

read_excel with sheetname=None or sheetname=[0,1,”Sheet5”] returns an OrderedDict instead of a dict

Follow-up of #9933

This still needs tests :). Take a look at the [excel tests](https://github.com/pydata/pandas/blob/d83bbff4f00915a4f30d367bf8e98487ff3f4503/pandas/io/tests/test_excel.py#L428) and update some or all of them to require sheets to appear in the original order.
 How can I run only this test ?
(because I don't want to run all tests using tests.sh)

I tried 

```
$ python pandas/io/tests/test_excel.py
cannot import name hashtable
Traceback (most recent call last):
  File "pandas/io/tests/test_excel.py", line 3, in <module>
    from pandas.compat import u, range, map, openpyxl_compat
  File "/Users/scls/github/scls19fr/pandas/pandas/__init__.py", line 7, in <module>
    from . import hashtable, tslib, lib
ImportError: cannot import name hashtable
```

I also tried

```
bash test_fast.sh
E
======================================================================
ERROR: Failure: ImportError (cannot import name hashtable)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "//anaconda/lib/python2.7/site-packages/nose/loader.py", line 414, in loadTestsFromName
    addr.filename, addr.module)
  File "//anaconda/lib/python2.7/site-packages/nose/importer.py", line 47, in importFromPath
    return self.importFromDir(dir_path, fqname)
  File "//anaconda/lib/python2.7/site-packages/nose/importer.py", line 94, in importFromDir
    mod = load_module(part_fqname, fh, filename, desc)
  File "/Users/scls/github/scls19fr/pandas/pandas/__init__.py", line 7, in <module>
    from . import hashtable, tslib, lib
ImportError: cannot import name hashtable

----------------------------------------------------------------------
Ran 1 test in 0.001s

FAILED (errors=1)
```
 @scls19fr Take a look at these sections of the contributing guidelines (might be worth reading other sections, too!):
http://pandas-docs.github.io/pandas-docs-travis/contributing.html#making-changes
http://pandas-docs.github.io/pandas-docs-travis/contributing.html#running-the-test-suite

(to be more specific, that error message means you need to rebuild pandas)
 I think this test could be good

```
def test_reading_all_sheets(self):
    # Test reading all sheetnames by setting sheetname to None,
    # Ensure a dict is returned.
    # See PR #9450
    # Ensure sheet order is preserved.
    # See issue #9930

    _skip_if_no_xlrd()

    dfs = read_excel(self.multisheet,sheetname=None)
    expected_keys = ['Alpha','Beta','Charlie']
    tm.assert_contains_all(expected_keys,dfs.keys())
    for i, sheetname in enumerate(dfs):
        expected_sheetname = expected_keys[i]
        tm.assert_equal(sheetname, expected_sheetname)
```

But I won't send a new PR until I can run test on my side.
Any idea ?
 I also wonder what should happen when a list is given as sheetname parameter. Should list order be preserved or should excel sheetname order be preserved ?
 I would preserve the excel sheetname order. The user already supplied a list of desired sheets, so they can access them in that order if desired. But if we reorder them to the supplied order, there is no way for them to access them in the original order.

This rule is also simpler to explain.
 Preserving order defined by user will be (in my mind) both much easier to explain and much easier to implement. I don't see how we can preserve excel sheetname order easily.

see `ExcelFile._parse_excel`

```
    if isinstance(sheetname, list):
        sheets = sheetname
        ret_dict = True
    elif sheetname is None:
        sheets = self.sheet_names
        ret_dict = True
    else:
        sheets = [sheetname]

....

for asheetname in sheets:
    ...
```
 @scls19fr you don't need to open a new pull requests each time you update. just push to the same one.
 Sorry @jreback I still need to learn Git and GitHub
 To preserve original sheet order, you could use something like this:

```
    if isinstance(sheetname, list):
        requested_sheets = set(sheetname)
        if not requested_sheets <= set(self.sheet_names):
            raise ValueError('...')
        sheets = [s for s in self.sheet_names if s in requested_sheets]
        ret_dict = True
```
 @scls19fr pls add some tests.
 I'm sorry but I'm quite busy these days
 closing if/when updated, pls reopen
