docs: pin down sphinx to avoid crash with Sphinx 3.0.0

docs: pin down sphinx to avoid crash with Sphinx 3.0.0

The vpp-make-test-docs-verify jobs started to fail. The last successful run of it shows:

reating file /w/workspace/vpp-make-test-docs-verify-master/build-root/build-test/doc/api/vpp_vxlan_gbp_tunnel.rst.
Creating file /w/workspace/vpp-make-test-docs-verify-master/build-root/build-test/doc/api/vpp_vxlan_tunnel.rst.
Creating file /w/workspace/vpp-make-test-docs-verify-master/build-root/build-test/doc/api/vrf.rst.
Creating file /w/workspace/vpp-make-test-docs-verify-master/build-root/build-test/doc/api/modules.rst.
sphinx-build -b html -d /w/workspace/vpp-make-test-docs-verify-master/build-root/build-test/doc/.sphinx-cache  /w/workspace/vpp-make-test-docs-verify-master/build-root/build-test/doc/api -c /w/workspace/vpp-make-test-docs-verify-master/test/doc /w/workspace/vpp-make-test-docs-verify-master/build-root/build-test/doc/html
Running Sphinx v2.4.4
making output directory... done
building [mo]: targets for 0 po files that are out of date
building [html]: targets for 161 source files that are out of date
updating environment: [new config] 161 added, 0 changed, 0 removed
reading sources... [  0%] bfd
reading sources... [  1%] debug
reading sources... [  1%] debug_internal
reading sources... [  2%] discover_tests

The failing jobs show:

Creating file /w/workspace/vpp-make-test-docs-verify-master/build-root/build-test/doc/api/vpp_vxlan_tunnel.rst.
Creating file /w/workspace/vpp-make-test-docs-verify-master/build-root/build-test/doc/api/vrf.rst.
Creating file /w/workspace/vpp-make-test-docs-verify-master/build-root/build-test/doc/api/modules.rst.
sphinx-build -b html -d /w/workspace/vpp-make-test-docs-verify-master/build-root/build-test/doc/.sphinx-cache  /w/workspace/vpp-make-test-docs-verify-master/build-root/build-test/doc/api -c /w/workspace/vpp-make-test-docs-verify-master/test/doc /w/workspace/vpp-make-test-docs-verify-master/build-root/build-test/doc/html
Running Sphinx v3.0.0
making output directory... done
building [mo]: targets for 0 po files that are out of date
building [html]: targets for 161 source files that are out of date
updating environment: [new config] 161 added, 0 changed, 0 removed
reading sources... [  0%] bfd

Exception occurred:
  File "/usr/lib/python3.6/inspect.py", line 516, in unwrap
    raise ValueError('wrapper loop when unwrapping {!r}'.format(f))
ValueError: wrapper loop when unwrapping scapy.fields.BitEnumField
The full traceback has been saved in /tmp/sphinx-err-n84dadfq.log, if you want to report the issue to the developers.
Please also report this if it was a user error, so that a better error message can be provided next time.
A bug report can be filed in the tracker at <https://github.com/sphinx-doc/sphinx/issues>. Thanks!
Makefile:39: recipe for target 'html' failed
make[2]: *** [html] Error 2
make[2]: Leaving directory '/w/workspace/vpp-make-test-docs-verify-master/test/doc'
Makefile:274: recipe for target '/w/workspace/vpp-make-test-docs-verify-master/build-root/build-test/doc' failed

Type: fix

Change-Id: Id98c0f94104e455ea819aacec62f605e53db13ce
Signed-off-by: Andrew Yourtchenko <ayourtch@gmail.com>