sqlite3 readonly issue

i installed aws-shell within a virtualenv and facing the following issue:

```
(venv) frits@frits-ThinkPad-E580:~/aws-shell$ aws-shell
Creating doc index in the background. It will be a few minutes before all documentation is available.
aws> Exception in thread Thread-1:
Traceback (most recent call last):
File "/usr/lib/python3.6/threading.py", line 916, in _bootstrap_inner
self.run()
File "/usr/lib/python3.6/threading.py", line 864, in run
self._target(*self._args, **self._kwargs)
File "/home/frits/aws-shell/venv/lib/python3.6/site-packages/awsshell/makeindex.py", line 88, in write_doc_index
do_write_doc_index(db, help_command, close_db_on_finish=should_close)
File "/home/frits/aws-shell/venv/lib/python3.6/site-packages/awsshell/makeindex.py", line 93, in do_write_doc_index
_index_docs(db, help_command)
File "/home/frits/aws-shell/venv/lib/python3.6/site-packages/awsshell/makeindex.py", line 110, in _index_docs
db[dotted_name] = text_docs
File "/home/frits/aws-shell/venv/lib/python3.6/site-packages/awsshell/db.py", line 43, in __setitem__
{'key': key, 'value': value})
sqlite3.OperationalError: attempt to write a readonly database

```

Start by reproducing the reported aws-shell run in the virtualenv, then read awsshell/makeindex.py and awsshell/db.py at the traceback locations. Determine why the document index database is readonly; done means the background index can write its entries without the sqlite3.OperationalError.

Written by the indexing model from the issue text.