Pycobertura

* BACKWARD INCOMPATIBLE: The class `Cobertura` no longer instantiates a default
`FileSystem` object if none is provided in the constructor
`Cobertura.__init__(filesystem=...)`. If the content of files is accessed
(e.g. to render a full coverage report) then a `FileSystem` instance must be
provided.
* The class `Cobertura` will raise `MissingFileSystem` if no
`FileSystem` object was provided via the keyword argument
`Cobertura(filesystem=...)`. It will only be raised when calling methods that
attempt to read the content of files, e.g. `Cobertura.file_source(...)` or
`Cobertura.source_lines(...)`.

* Support loading Cobertura reports from an XML string. Thanks williamfzc
* Fix (or add?!) support for providing file objects to `Cobertura`.

* Fix misreported coverage when a single coverage file is used with
`pycobertura diff` to overlay covered and uncovered changes between two
different sources. Thanks bastih

* Let the caller customize the appearance of the HTML report providing a
`title`, omitting the rendering of sources by means of the boolean
`render_file_sources` and providing an helpful message to the end-users (in
place of the sources) by means of the `no_file_sources_message` parameter.
Contributed by nilleb.
* Add a `GitFilesystem` to allow pycobertura to access source files at different
revisions from a git repository. Thanks nilleb.
* BACKWARDS INCOMPATIBLE: Change the signature of the Cobertura object in order
to accept a filesystem.
* BACKWARDS INCOMPATIBLE: Drop support for Python 2.
* Added tox task `black`, to use the the uncompromising Python code formatter.
See https://black.readthedocs.io/en/stable/ to learn more about black. Thanks
nilleb.

* Use a different `memoize()` implementation so that cached objects can be
freed/garbage collected and prevent from running out of memory when processing
a lot of cobertura files. Thanks kannaiah

* Calculate the correct line rate for diffs (83). Previously
`CoberturaDiff.diff_line_rate` with no filename argument would total up the
different line rate changes from all of the modified files, which is not the
correct difference in line rates between all files. Now the difference in line
rate from the two reports objects will be directly used if no argument is
passed. (borgstrom)