Fix the following 2 errors when running on Python 3.8
```
Traceback (most recent call last):
File "<string>", line 1, in <module>
File "/home/yen/Projects/nvchecker/nvchecker/main.py", line 33, in main
if core.process_common_arguments(args):
File "/home/yen/Projects/nvchecker/nvchecker/core.py", line 46, in process_common_arguments
nicelogger.enable_pretty_logging(
File "/home/yen/Projects/nvchecker/nvchecker/lib/nicelogger.py", line 93, in enable_pretty_logging
formatter = TornadoLogFormatter(color=color)
File "/home/yen/Projects/nvchecker/nvchecker/lib/nicelogger.py", line 16, in __init__
super().__init__(self, *args, **kwargs)
File "/usr/lib/python3.8/logging/__init__.py", line 589, in __init__
self._style.validate()
File "/usr/lib/python3.8/logging/__init__.py", line 441, in validate
if not self.validation_pattern.search(self._fmt):
TypeError: expected string or bytes-like object
```
```
Traceback (most recent call last):
File "<string>", line 1, in <module>
File "/home/yen/Projects/nvchecker/nvchecker/main.py", line 42, in main
ioloop.run_until_complete(s.check())
File "/usr/lib/python3.8/asyncio/base_events.py", line 589, in run_until_complete
return future.result()
File "/home/yen/Projects/nvchecker/nvchecker/core.py", line 177, in check
self.print_version_update(name, result)
File "/home/yen/Projects/nvchecker/nvchecker/core.py", line 190, in print_version_update
logger.info('updated', name=name, version=version, old_version=oldver)
File "/home/yen/.local/lib/python3.8/site-packages/structlog/_base.py", line 191, in _proxy_to_logger
args, kw = self._process_event(method_name, event, event_kw)
File "/home/yen/.local/lib/python3.8/site-packages/structlog/_base.py", line 151, in _process_event
event_dict = proc(self._logger, method_name, event_dict)
File "/home/yen/Projects/nvchecker/nvchecker/slogconf.py", line 48, in stdlib_renderer
getattr(logger, level)(
File "/usr/lib/python3.8/logging/__init__.py", line 1441, in info
self._log(INFO, msg, args, **kwargs)
File "/usr/lib/python3.8/logging/__init__.py", line 1572, in _log
fn, lno, func, sinfo = self.findCaller(stack_info, stacklevel)
TypeError: findCaller() takes from 1 to 2 positional arguments but 3 were given
```
To fix the second issue, I backported relevant CPython changes [1] to slogconf.py
Test command: `python3.8 -c 'from nvchecker.main import main; main()' sample_source.ini`
[1] dde9fdbe45 (diff-c11fc567cf5b6e518eb855c23b5599bcR1400)
