ENH: adding support for Py3.6+ memory tracing for khash-maps
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Tracemalloc is  a standard tool for tracing of memory consumption in Python. Until now khash-maps were invisible for this tool.

This PR follows more or less numpy's approach (see https://github.com/numpy/numpy/commit/03534ec90dba2bcdcd649be64be57939dde4c6f5) for adding  tracemalloc-support.

This PR fixes also silly mistakes from #37920 (see https://github.com/pandas-dev/pandas/pull/38048/commits/3291ed198432a80eb559382fbf9a35cb6a8ad749) and the somewhat imprecise implementation of `sizeof` for `XXXHashTable`-classes.

