Slack pack won't install on python 3.8

[amanda11]

The slack pack won't install on 3.5dev on focal and complains first with:

      1 | #include "libxml/xpath.h"
        |          ^~~~~~~~~~~~~~~~
  compilation terminated.
  *********************************************************************************
  Could not find function xmlCheckVersion in library libxml2. Is libxml2 installed?

Installing libxml2-dev and libxslt1-dev resolves this problem, but then the following problem is seen:

    x86_64-linux-gnu-gcc -pthread -Wno-unused-result -Wsign-compare -DNDEBUG -g -fwrapv -O2 -Wall -g -fstack-protector-strong -Wformat -Werror=format-security -g -fwrapv -O2 -g -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2 -fPIC -I/usr/include/libxml2 -Isrc/lxml/includes -I/opt/stackstorm/virtualenvs/slack/include -I/usr/include/python3.8 -c src/lxml/lxml.etree.c -o build/temp.linux-x86_64-3.8/src/lxml/lxml.etree.o -w
    src/lxml/lxml.etree.c: In function __Pyx_PyCFunction_FastCall:
    src/lxml/lxml.etree.c:233279:13: error: too many arguments to function (PyObject * (*)(PyObject *, PyObject * const*, Py_ssize_t))meth
    233279 |     return (*((__Pyx_PyCFunctionFast)meth)) (self, args, nargs, NULL);
           |            ~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    src/lxml/lxml.etree.c: In function __Pyx_PyDict_Items:
    src/lxml/lxml.etree.c:4532:83: error: too many arguments to function (PyObject * (*)(PyObject *, PyObject * const*, Py_ssize_t))__pyx_umethod_PyDict_Type_items.func

lxml < 4.0.0 doesn't support python 3.8, and so for focal need to use newer version of lxml.

[amanda11]

Slack pack installs with newer lxml. Checked other dependencies, and slack client library version used states highest version supported in python3.6. Although it installs on python3.6, would suggest its also upgraded.