Force xlsm files to open with pyexcel-xlsx

`.xlsm` files are like `.xlxs` files but with macros enabled. They store
data in the same XML-based format as `.xlsx` files.

Pyexcel will try to use the xlrd package to parse `.xlsm` files. This
used to work because xlrd used to support reading `.xlsx` files. xlrd
has dropped support for `.xlsx` files in version 2 because of security
concerns. This means that when pyexcel asks xlrd to parse a `.xlsm` file
it causes an error.

This commit adds some branching to force `.xlsm` files to be opened
with pyexcel-xlsx instead, which does support `.xlsx` files.

app/models/spreadsheet.py

@@ -3,6 +3,7 @@ from io import StringIO
 from os import path
 
 import pyexcel
+import pyexcel_xlsx
 
 
 class Spreadsheet():
@@ -72,6 +73,15 @@ class Spreadsheet():
             file_content = StringIO(
                 Spreadsheet.normalise_newlines(file_content))
 
+        if extension == 'xlsm':
+            file_data = pyexcel_xlsx.get_data(file_content)
+            instance = cls.from_rows(
+                # Get the first sheet from the workbook
+                list(file_data.values())[0],
+                filename,
+            )
+            return instance
+
         instance = cls.from_rows(
             pyexcel.iget_array(
                 file_type=extension,