Add command that creates a .whl package for Python, current modern
format to be installed into virtual environmants

Update script for make target pythemis_make_wheel, remove venv path from
PATH if detected, so that building wheel always works properly.

Add make target pythemis_install_wheel to install created wheel into
currently active venv.

Update CI config to
1) Use Ubuntu 20.04 (since the're some issues with themispp tests)
2) Use new pythemis_install_wheel command instead of hardcoded filename

Rename pythemis_install -> pythemis_old_install here as well

Use wheel content to create the .deb package.
Add CI job to test with this specific installation method.

* Rename pythemis_deb -> deb_python
* Remove some comments, TODOs
* Add libthemis to PyThemis deb deps

Since python3-pythemis now depends on libthemis, we need to install the
dependency before installing Python wrapper.

Updated Makefile to conditionally include C++ and Java wrappers in deb
target so that one could build only Themis itself on systems without
Java (jni.h is required to to build Java wrapper). This may not be the
best way, but it uses the least amount of changes.

Potential improvements:
* Split deb target into deb_themis, deb_themispp, deb_java (will need to
  copy-paste some vars as well, and to the same to rpm for consistency)
* Add new target like deb_themis_install and others, just like did it
  with pythemis_install_deb

Last version that still works with current MSRV 1.58.
New ones want 1.60 or newer.

Same reason as with 'log', to avoid increasing MSRV when only tests
depend on this.