Replace bare `except:` with `except Exception`, there are some cases where it
can be narrowed further, but this is already an improvement over the current
situation.

This is somewhat pursuant to eliminating
[flakeheaven](https://github.com/flakeheaven/flakeheaven), as it no longer
supports the latest version of flake8
[[ref](https://github.com/flakeheaven/flakeheaven/issues/132)]. But it also is
just the right thing to do as bare exceptions can cause problems.

Disable
[`implicit_reexport`](https://mypy.readthedocs.io/en/stable/config_file.html#confval-implicit_reexport)
and eliminate all errors reported by mypy after this.

This helps ensure that import statements import from the right module and that
the `__all__` variable is correct.

Add optional keyword only `target_graph` argument to `rdflib.graph.Graph.cbd` and use this new argument in `evalDescribeQuery`.

This makes it possible to compute a concise bounded description without creating a new graph to hold the result, and also without potentially having to copy it to another final graph.

Co-authored-by: Iwan Aucamp <aucampia@gmail.com>

This change removes the redundant inheritance from `object` (i.e. `class
Foo(object): pass`) that is no longer needed in Python 3 and is a relic from
Python 2.

This change narrows import so that things are imported from the Python module
where they are defined instead of importing them from a module that re-exports
them, e.g. change import of `Graph` to import from the `rdflib.graph` module
instead of from the `rdflib` module. This helps avoid problems with circular
imports.

`XMLNS` was seen as unused in `rdflib/__init__.py` but this change adds it to
`__all__` in that module so that it is not unused.

This change also removes actual unused imports in
`rdflib/namespace/__init__.py`.

Compatibility handling for `collections.abc.Mapping` and
`collections.abc.MutableMapping` is not needed as RDFLib currently only support
Python 3.7 and newer, and those classes are available from `collections.abc` in
Python 3.7.

This change eliminates some situations where a mutable object (i.e., a dictionary) was used as the default value for functions in the `rdflib.plugins.sparql.processor` module and related code. It replaces these situations with `typing.Optinal` that defaults to None, and is then handled within the function. Luckily, some of the code that the SPARQL Processor relied on already had this style, meaning not a lot of changes had to be made.

This change also makes a small update to the logic in the SPARQL Processor's query function to simplify the if/else statement. This better mirrors the implementation in the `UpdateProcessor`.

Change the type-hints for `Graph.__contains__` to also accept `Path`
values as the parameter is passed to the `Graph.triples` function,
which accepts `Path` values.

Previously, `rdflib.plugins.sparql.algebra.translateAlgebra()` maintained state via a file, with a fixed filename `query.txt`.  With this change, use of that file is eliminated; state is now maintained in memory so that multiple concurrent `translateAlgebra()` calls, for example, should no longer interfere with each other.

The change is accomplished with no change to the client interface.  Basically, the actual functionality has been moved into a class, which is instantiated and used as needed (once per call to `algrebra.translateAlgebra()`).