Update testcode module to python3 (Fixes: #199)

This is the result of three things:

1. running 2to3 in the testcode/bin and testcode/lib directories
2. changing the shebang of testcode/bin/testcode.py from python2 to python3
3. revert 4243627f

Took me quite a long time of debugging to figure out where the testsuite was stuck, the python3 process was looping at 100%cpu and constantly eating memory.

It was stuck in this code in __init__.py and looping endlessly.

                    for line in iter(extract_popen.stdout.readline, ''):
                      #print line,
                      lines.append(line)      

After I reverted the above commit, it works fine for me, but it should be tested independently probably.

This patch has now been added to the Debian package, as the python2 removal is coming soon:

https://salsa.debian.org/debichem-team/espresso/-/blob/master/debian/patches/testsuite_python3.patch