Deprecation warnings in Python 3.8

[jaraco]

Here are some of the deprecation warnings emitted when running tests on Python 3.8:

/Users/jaraco/code/reubano/meza/.tox/py38/lib/python3.8/site-packages/manager/main.py:3: DeprecationWarning: the imp module is deprecated in favour of importlib; see the module's documentation for alternative uses
  import imp
/Users/jaraco/code/reubano/meza/.tox/py38/lib/python3.8/site-packages/manager/__init__.py:65: DeprecationWarning: inspect.getargspec() is deprecated since Python 3.0, use inspect.signature() or inspect.getfullargspec()
  self.arg_names, varargs, keywords, defaults = inspect.getargspec(
...
/Users/jaraco/code/reubano/meza/.tox/py38/lib/python3.8/site-packages/nose/plugins/manager.py:418: DeprecationWarning: pkg_resources is deprecated as an API. See https://setuptools.pypa.io/en/latest/pkg_resources.html
  import pkg_resources
/Users/jaraco/code/reubano/meza/.tox/py38/lib/python3.8/site-packages/nose/importer.py:12: DeprecationWarning: the imp module is deprecated in favour of importlib; see the module's documentation for alternative uses
  from imp import find_module, load_module, acquire_lock, release_lock
/Users/jaraco/code/reubano/meza/.tox/py38/lib/python3.8/site-packages/nose/suite.py:106: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated since Python 3.3, and in 3.10 it will stop working
  if isinstance(tests, collections.Callable) and not is_suite:
#140 Test for reading open files ... /Users/jaraco/code/reubano/meza/tests/test_io.py:379: DeprecationWarning: 'U' mode is deprecated
  f = open(filepath, "rU", newline=None)
...

All but one of these warnings is coming from nose and manage.py, both projects which appear to be abandoned. These deprecation warnings fully break on later Pythons. Let's find replacement for these libraries (I suggest tox and pytest may be sufficient).