WIP: configure: Add checking python3 and python3-sphinx
This MR contains two updates.
First, add checking python3 in aclocal.m4 and configure.ac
We need python3 instead of python2 for the following reasons:
- To build documentation with python3-sphinx
- To run testsuites
- To deal with python2 expiration at the end of 2019
Second, add checking python3-sphinx in configure.ac
We need python3-sphinx instead of python2-sphinx to build documentation. It is difficult to distinguish directly between sphinx for python3 and python2. Therefore, this commit tests by trying import as follows:
python3 -c "import sphinx"
If sphinx for python2 is installed, it can be detected as an error.
See also #17346 (closed) and #17356 (closed)