Add typing support

[enzo-santos]

This PR adds typing support to filecache, turning it into a type-safe library.

It is type-checked using mypy:

$ python -m mypy --version
mypy 1.6.0 (compiled: yes)
$ python -m mypy filecache
Success: no issues found in 3 source files

It also passes all tests:

$ python -m filecache.test.test_filecache
\filecache\filecache\test\test_filecache.py:5: DeprecationWarning: the imp module is deprecated in favour of importlib and slated for removal in Python 3.12; see the module's documentation for alternative uses
  import imp
erasing \filecache\filecache\test\stub_for_test.py.cache.bak
erasing \filecache\filecache\test\stub_for_test.py.cache.dat
erasing \filecache\filecache\test\stub_for_test.py.cache.dir
erasing \filecache\filecache\test\test_filecache.py.cache.bak
erasing \filecache\filecache\test\test_filecache.py.cache.dat
erasing \filecache\filecache\test\test_filecache.py.cache.dir
erasing \filecache\_lt_string_gt_.cache.bak
erasing \filecache\_lt_string_gt_.cache.dat
erasing \filecache\_lt_string_gt_.cache.dir
.........
----------------------------------------------------------------------
Ran 9 tests in 0.807s

OK

This implementation transfers all the caching logic into the CachedFileCacheCallable class, which is callable and acts like a function. This makes the filecache decorator a lot more clean.

The typing implementation is based on https://github.com/maxfischer2781/asyncstdlib/blob/master/asyncstdlib/_lrucache.pyi. It makes the filecache decorator type-check on both plain functions and methods.

Feel free to change as desired, specially variable naming.