BUG: read_excel return empty dataframe when using usecols

- [x] closes #18273
- [x] tests added / passed
- [x] passes git diff master --name-only -- "*.py" | grep "pandas/" | xargs -r flake8
- [x] whatsnew entry

As mentioned read_excel returns an empty DataFrame when usecols argument is a list of strings.
Now lists of strings are correctly interpreted by read_excel function.

Author: jacksonjos

pandas/io/excel.py

@@ -477,8 +477,15 @@ def _excel2num(x):
 
         if isinstance(usecols, int):
             return i <= usecols
+        # check if usecols is a string that indicates a comma separated list of
+        # Excel column letters and column ranges
         elif isinstance(usecols, compat.string_types):
             return i in _range2cols(usecols)
+        # check if usecols is a list of strings, each one indicating a Excel
+        # column letter or a column range
+        elif all(isinstance(x, compat.string_types) for x in usecols):
+            usecols_str = ",".join(usecols)
+            return i in _range2cols(usecols_str)
         else:
             return i in usecols