ReqIF export/import improvements

What's Changed

• Bump version 0.0.54 by @stanislaw in #1776
• Technical debt: document_screen_view_object: encapsulate render moved node update by @stanislaw in #1778
• Composable Documents: copy assets in a redundant way for included documents by @stanislaw in #1780
• Bump version to 0.0.55a1 by @stanislaw in #1781
• Code climate: fix mypy version guard by @stanislaw in #1782
• Code climate: enable attr-defined and no-any-return check by @stanislaw in #1783
• Code climate: enable remaining mypy checks by @stanislaw in #1784
• Code climate: sdoc/models/document: fix no-untyped-call by @stanislaw in #1785
• Code climate: sdoc/models/document: fix no-untyped-def by @stanislaw in #1786
• Code climate: sdoc/models/section: remove unused code by @stanislaw in #1787
• backend/reqif: reqif_to_sdoc: simplify iteration by @stanislaw in #1789
• backend/reqif: reqif_to_sdoc: simplify iteration further by @stanislaw in #1791
• CI: drop Python 3.7 from macOS jobs by @stanislaw in #1793
• backend/reqif: export: --multiline-is-xhtml option by @stanislaw in #1792
• backend/reqif: reqif_to_sdoc_converter.py: fix arg-type issues by @stanislaw in #1794
• backend/reqif: reqif_to_sdoc_converter.py: fix no-untyped-call issues by @stanislaw in #1795
• backend/reqif: fix the XHTML namespace by @stanislaw in #1796
• backend/reqif: update to the latest ReqIF library by @stanislaw in #1797
• pickle: catch AttributeErrors on schema changes, raise AssertionError otherwise by @stanislaw in #1798
• pickle_cache: reuse between Document and DocumentGrammar for caching both by @stanislaw in #1799
• backend/reqif: --reqif-enable-mid option to allow bi-directional IDENTIFIER-MID mapping by @stanislaw in #1800
• backend/reqif: --reqif-import-markup option to specify HTML or other options when importing by @stanislaw in #1802

Full Changelog: 0.0.54...0.0.55