Ensure compatibility with Python 3.11

[dtf0]

• Replace deprecated getargspec() with getfullargspec()
• Replace deprecated formatargspec() with custom implementation

Copied formatconfigargspec() from
sicherha/powerline@6238d9e after a number of
failed attempts to use inspect.signature or Signature directly as suggested.

Resolves errors in Python 3.11 & warnings in 3.10:

$ python3.11 zdeskcfg.py
Traceback (most recent call last):
  File "zdeskcfg/zdeskcfg.py", line 161, in <module>
    @configure()
     ^^^^^^^^^^^
  File "zdeskcfg/zdeskcfg.py", line 36, in __call__
    tgt_argspec = inspect.getargspec(tgt_func)
                  ^^^^^^^^^^^^^^^^^^
AttributeError: module 'inspect' has no attribute 'getargspec'. Did you mean: 'getargs'?

$ python3.10 zdeskcfg.py
zdeskcfg/zdeskcfg.py:36: DeprecationWarning: inspect.getargspec() is deprecated since Python 3.0, use inspect.signature() or inspect.getfullargspec()
  tgt_argspec = inspect.getargspec(tgt_func)
zdeskcfg/zdeskcfg.py:42: DeprecationWarning: inspect.getargspec() is deprecated since Python 3.0, use inspect.signature() or inspect.getfullargspec()
  argspec = inspect.getargspec(tgt_func)
zdeskcfg/zdeskcfg.py:53: DeprecationWarning: `formatargspec` is deprecated since Python 3.5. Use `signature` and the `Signature` object directly
  signature = inspect.formatargspec(
zdeskcfg/zdeskcfg.py:66: DeprecationWarning: `formatargspec` is deprecated since Python 3.5. Use `signature` and the `Signature` object directly
  newsignature = inspect.formatargspec(*newargspec)[1:-1]