admin/doc-requirements: use funcparserlib from github

admin/doc-requirements: use funcparserlib from github

funcparserlib is pulled in as a dependency by blockdiag. the latest version of
funcparserlib available on pypi is v0.3.6 which is not compatible with
Python3.8.

in this change, funcparserlib is installed from github instead to
address the build failure like:

  File "/home/docs/checkouts/readthedocs.org/user_builds/ceph/envs/41855/lib/python3.8/site-packages/sphinxcontrib/seqdiag.py", line 26, in <module>
    import seqdiag.utils.rst.nodes
  File "/home/docs/checkouts/readthedocs.org/user_builds/ceph/envs/41855/lib/python3.8/site-packages/seqdiag/utils/rst/nodes.py", line 16, in <module>
    from blockdiag.utils.rst import nodes
  File "/home/docs/checkouts/readthedocs.org/user_builds/ceph/envs/41855/lib/python3.8/site-packages/blockdiag/utils/rst/nodes.py", line 21, in <module>
    import blockdiag.builder
  File "/home/docs/checkouts/readthedocs.org/user_builds/ceph/envs/41855/lib/python3.8/site-packages/blockdiag/builder.py", line 16, in <module>
    from blockdiag import parser
  File "/home/docs/checkouts/readthedocs.org/user_builds/ceph/envs/41855/lib/python3.8/site-packages/blockdiag/parser.py", line 43, in <module>
    from funcparserlib.parser import (a, finished, forward_decl, many, maybe, skip,
  File "/home/docs/checkouts/readthedocs.org/user_builds/ceph/envs/41855/lib/python3.8/site-packages/funcparserlib/parser.py", line 123
    except NoParseError, e:
                       ^
SyntaxError: invalid syntax

once https://github.com/vlasovskikh/funcparserlib/issues/65 is
addressed, we should drop this change.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>