Start adding types to Submodule, add py.typed to manifest - #1282

Returning an explicit value from a generator function causes that
value to be bound to the `value` attribute of the StopIteration
exception. This is available as the result of "yield from" when it
is used as an expression; or by explicitly catching StopIteration,
binding the StopIteration exception to a variable, and accessing
the attribute. This feature of generators is rarely used.

The `return iter([])` statement in Submodule.iter_items uses this
feature, causing the resulting StopIteration exception object to
have a `value` attribute that refers to a separate second iterator
that also yields no values. From context, this behavior is clearly
not the goal; a bare return statement should be used here (which
has the same effect except for the `value` attribute of the
StopIteration exception). The code had used a bare return prior to
82b131c (gitpython-developers#1282), when `return` was changed to `return iter([])`.
That was part of a change that added numerous type annotations. It
looks like it was either a mistake, or possibly an attempt to work
around an old bug in a static type checker.

This commit extends the test_iter_items_from_invalid_hash test to
assert that the `value` attribute of the StopIteration is its usual
default value of None. This commit only extends the test; it does
not fix the bug.

Returning an explicit value from a generator function causes that
value to be bound to the `value` attribute of the StopIteration
exception. This is available as the result of "yield from" when it
is used as an expression; or by explicitly catching StopIteration,
binding the StopIteration exception to a variable, and accessing
the attribute. This feature of generators is rarely used.

The `return iter([])` statement in Submodule.iter_items uses this
feature, causing the resulting StopIteration exception object to
have a `value` attribute that refers to a separate second iterator
that also yields no values. From context, this behavior is clearly
not the goal; a bare return statement should be used here (which
has the same effect except for the `value` attribute of the
StopIteration exception). The code had used a bare return prior to
82b131c (gitpython-developers#1282), when `return` was changed to `return iter([])`.
That was part of a change that added numerous type annotations. It
looks like it was either a mistake, or possibly an attempt to work
around an old bug in a static type checker.

This commit extends the test_iter_items_from_invalid_hash test to
assert that the `value` attribute of the StopIteration is its usual
default value of None. This commit only extends the test; it does
not fix the bug.

Returning an explicit value from a generator function causes that
value to be bound to the `value` attribute of the StopIteration
exception. This is available as the result of "yield from" when it
is used as an expression; or by explicitly catching StopIteration,
binding the StopIteration exception to a variable, and accessing
the attribute. This feature of generators is rarely used.

The `return iter([])` statement in Submodule.iter_items uses this
feature, causing the resulting StopIteration exception object to
have a `value` attribute that refers to a separate second iterator
that also yields no values (gitpython-developers#1779).

From context, this behavior is clearly not the goal; a bare return
statement should be used here (which has the same effect except for
the `value` attribute of the StopIteration exception). The code had
used a bare return prior to 82b131c (gitpython-developers#1282), when `return` was
changed to `return iter([])`. That was part of a change that added
numerous type annotations. It looks like it was either a mistake,
or possibly an attempt to work around an old bug in a static type
checker.

This commit extends the test_iter_items_from_invalid_hash test to
assert that the `value` attribute of the StopIteration is its usual
default value of None. This commit only extends the test; it does
not fix the bug.