Clean up python bytecode from doc build

Otherwise they escape into the release tarball.

(cherry picked from commit 424bed7b2943e0bc32a3feab4c55a4ea056bfa21)

https://github.com/krb5/krb5/commit/8a3d375f6b85db6faa223b207d1970cffe85842a
Author: Ben Kaduk <kaduk@mit.edu>
Committer: Tom Yu <tlyu@mit.edu>
Commit: 8a3d375f6b85db6faa223b207d1970cffe85842a
Branch: krb5-1.11
 src/doc/Makefile.in |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)