Fix bug regarding multiline docstrings

[jonafato]

If a multiline docstring has its closing quotes at the end of a line
instead of on a separate line, isort fails to properly add imports using
the add_imports config option; instead, it adds the desired imports
into the middle of the docstring as illustrated below. While PEP 257
(and other guides) advises that closing quotes appear on their own line,
isort should not fail here.

This change adds a check for closing docstrings at the end of a line in
addition to the existing line start check for all comment indicators. A
new section of the test_add_imports test explicitly tests multiline
imports and this failure scenario specifically.

---

A working example:

"""My module.

Provides example functionality.
"""

print("hello, world")

Running isort --add-import "from __future__ import annotations"
produces the following as expected:

"""My module.

Provides example functionality.
"""

from __future__ import annotations

print("hello, world")

---

The failure behavior described:

"""My module.

Provides example functionality."""

print("hello, world")

Running isort --add-import "from __future__ import annotations" as
above produces the following result:

"""My module.

from __future__ import annotations

Provides example functionality."""

print("hello, world")

Subsequent executions add more import lines into the docstring. This
behavior occurs even if the file already has the desired imports.

[timothycrosley]

Thanks for finding and fixing this issue!