doc: require Sphinx 3.4.3

doc: require Sphinx 3.4.3

For enums documented according to the requirements in chapter
"Structure, union, and enumeration documentation" of
https://www.kernel.org/doc/html/latest/doc-guide/kernel-doc.html
errors occur with Sphinx 2.4.4 which disappear with Sphinx 3.4.3,
e.g.
/builds/u-boot/custodians/u-boot-efi/doc/api/sysreset:6:
./include/sysreset.h:60:Unknown interpreted text role "enum".

Sphinx 3.4.3 is the version used by Debian in the current release.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>