Add {encode,decode}_filter_properties() functions to lzma module

Doc/library/lzma.rst

 :mod:`lzma` --- Compression using the LZMA algorithm
 ====================================================
 
 .. module:: lzma
    :synopsis: A Python wrapper for the liblzma compression library.
 .. moduleauthor:: Nadeem Vawda <nadeem.vawda@gmail.com>
 .. sectionauthor:: Nadeem Vawda <nadeem.vawda@gmail.com>
 
 .. versionadded:: 3.3
 
@@ skipping 215 matching lines @@
 -------------
 
 .. function:: check_is_supported(check)
 
    Returns true if the given integrity check is supported on this system.
 
    :const:`CHECK_NONE` and :const:`CHECK_CRC32` are always supported.
    :const:`CHECK_CRC64` and :const:`CHECK_SHA256` may be unavailable if you are
    using a version of :program:`liblzma` that was compiled with a limited
    feature set.
+
+
+.. function:: encode_filter_properties(filter)
+
+   Return a :class:`bytes` object encoding the options (properties) of the
+   filter specified by *filter* (a dictionary).
+
+   *filter* is interpreted as a filter specifier, as described in
+   :ref:`filter-chain-specs`.
+
+   The returned data does not include the filter ID itself, only the options.
+
+   This function is primarily of interest to users implementing custom file
+   formats.
+
+
+.. function:: decode_filter_properties(filter_id, encoded_props)
+
+   Return a dictionary describing a filter with ID *filter_id*, and options
+   (properties) decoded from the :class:`bytes` object *encoded_props*.
+
+   The returned dictionary is a filter specifier, as described in
+   :ref:`filter-chain-specs`.
+
+   This function is primarily of interest to users implementing custom file
+   formats.
 
 
 .. _filter-chain-specs:
 
 Specifying custom filter chains
 -------------------------------
 
 A filter chain specifier is a sequence of dictionaries, where each dictionary
 contains the ID and options for a single filter. Each dictionary must contain
 the key ``"id"``, and may contain additional keys to specify filter-dependent
@@ skipping 94 matching lines @@
 
 Creating a compressed file using a custom filter chain::
 
    import lzma
    my_filters = [
        {"id": lzma.FILTER_DELTA, "dist": 5},
        {"id": lzma.FILTER_LZMA2, "preset": 7 | lzma.PRESET_EXTREME},
    ]
    with lzma.LZMAFile("file.xz", "w", filters=my_filters) as f:
        f.write(b"blah blah blah")