docs/version.txt: updated release date to 2022-06-16.

setup.py: updates for 1.20.0rc2

Updated version to 1.20.0rc2 and default to use mupdf-1.20.0 release.

Reworked control of how to build mupdf:
    To force build to use system mupdf, set PYMUPDF_SETUP_MUPDF_BUILD to empty
    string.

    Renamed environmental variable PYMUPDF_SETUP_MUPDF_SDIST to
    PYMUPDF_SETUP_MUPDF_TGZ.

    Fixed use of MuPDF Memento builds on OpenBSD.

Bump version to 1.20.0rc1

upload v1.19.6

upload v1.19.5

correct changes documentation

upload v1.19.3

Merge branch 'master' of https://github.com/rk700/PyMuPDF

Update symbols.txt

Update README.md