Drop support for Python 3.6 (#1004)

* Drop support for Python 3.6
* Remove unused time parsing function
* Use Python 3.7 datetime function for writing stats data conversion
* Test against Python 3.10 on macOS and Windows

setup.cfg

@@ -11,7 +11,6 @@ license_file = LICENSE.md
 license = GNU General Public License v3
 classifiers =
     Programming Language :: Python :: 3 :: Only
-    Programming Language :: Python :: 3.6
     Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
     Programming Language :: Python :: 3.9
@@ -29,7 +28,7 @@ project_urls =
     Source Code = https://github.com/vkbo/novelWriter
 
 [options]
-python_requires = >=3.6
+python_requires = >=3.7
 include_package_data = True
 packages = find:
 install_requires =