OCRmyPDF

**Changes** - PDF/A made without Ghostscript ("speculative" conversion) is now validated with pikepdf's PDF/A support (`pikepdf.pdfa`) instead of veraPDF, and the final output is validated again after the metadata and optimization steps. veraPDF is no longer used, so the Ghostscript-free path is available whether or not veraPDF is installed, including in the Docker image. Files the validator does not approve go to Ghostscript as before; run with `-v1` to see why. - Speculative conversion now supports PDF/A-1b, and repairs common problems that previously always sent a file to Ghostscript: it replaces other output intents with sRGB, removes image interpolation flags, sets the Print flag on annotations, adds the `/CIDSet` PDF/A-1 requires, and removes XMP properties and hidden annotations that PDF/A does not permit. - New option `--pdfa-backend {auto,ghostscript,internal}` (API: `pdfa_backend=`). `auto`, the default, tries OCRmyPDF's own conversion and then Ghostscript. `internal` never uses Ghostscript, so JPEGs pass through unchanged; if the result is not approved, the `pdfa` output types fail with exit code 10 and `--output-type auto` outputs a regular