Fix return type of QTableWidget.cellWidget (#198)

`cellWidget` returns a `QWidget` if it exists, or `None` if it doesn't
exist.

Confirmed by this short script:

```python
from PyQt5 import QtCore, QtWidgets

print(f'Qt {QtCore.QT_VERSION_STR}')
print(f'PyQt {QtCore.PYQT_VERSION_STR}')

app = QtWidgets.QApplication(['foo'])
table = QtWidgets.QTableWidget(1, 1)

# empty cell inside table
print(table.cellWidget(0, 0))

# filled cell inside table
table.setCellWidget(0, 0, QtWidgets.QWidget())
print(table.cellWidget(0, 0))

# cell outside table
print(table.cellWidget(99, 99))
```

Output:

```
Qt 5.15.2
PyQt 5.15.6
None
<PyQt5.QtWidgets.QWidget object at 0x7fa13208bca0>
None
```

Author: altendky

CHANGELOG.md

@@ -5,12 +5,15 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 ## To be released
+
+### Added
 * [#192](https://github.com/python-qt-tools/PyQt5-stubs/pull/192) Add missing platform specific stubs:
   * Windows specific: QAxContainer.pyi QtWinExtras.pyi QtCore.QWinEventNotifier
   * MacOs X specific: QtMacExtras.pyi QtWidgets.QMacCocoaViewContainer
 
+### Changed
+* [#198](https://github.com/python-qt-tools/PyQt5-stubs/pull/198) Corrected `QTableWidget.cellWidget()` to return an an optional `QWidget` instead of a list of `QWidgets`.
 
-### Added
 ## 5.15.6.0
 
 ### Added

PyQt5-stubs/QtWidgets.pyi

@@ -10637,7 +10637,7 @@ class QTableWidget(QTableView):
     def setRangeSelected(self, range: QTableWidgetSelectionRange, select: bool) -> None: ...
     def removeCellWidget(self, arow: int, acolumn: int) -> None: ...
     def setCellWidget(self, row: int, column: int, widget: QWidget) -> None: ...
-    def cellWidget(self, row: int, column: int) -> typing.List[QWidget]: ...
+    def cellWidget(self, row: int, column: int) -> typing.Optional[QWidget]: ...
     def closePersistentEditor(self, item: typing.Optional[QTableWidgetItem]) -> None: ...  # type: ignore[override]
     def openPersistentEditor(self, item: typing.Optional[QTableWidgetItem]) -> None: ...  # type: ignore[override]
     def editItem(self, item: typing.Optional[QTableWidgetItem]) -> None: ...