Merge: Add compatibility to Python 3 (and retain Python 2)

Merge branch 'py6' of https://github.com/mlell/redo

* 'py6' of https://github.com/mlell/redo:
  Remove python<3.0 restriction in setup.py
  Make compatible to BeautifulSoup4
  Accept octal representations of Python 2 (0nnn) and Python 3 (0onnn)
  Prevent iterator being changed while iterating
  Python 2/3 compatible treatment of max(n, None)
  Prevent "Exception ... ignored" in `redo-log ... | head`
  Distinguish byte (python2 str type) and unicode strings (python 3 str type)
  Set file descriptor as inheritable for all pythons >=3.4
  Unify print function usage for Python 2 and 3 via __future__ import
  Run 2to3 utility
  Remove python interpreter selection

docs/md2man.py

@@ -1,5 +1,11 @@
+from __future__ import print_function
 import sys, os, markdown, re
-from BeautifulSoup import BeautifulSoup
+try:
+    from BeautifulSoup import BeautifulSoup
+    bsver = 3
+except ModuleNotFoundError:
+    from bs4 import BeautifulSoup
+    bsver = 4
 
 def _split_lines(s):
     return re.findall(r'([^\n]*\n?)', s)
@@ -179,7 +185,10 @@ def do_definition(tag):
 
 def do_list(tag):
     for i in tag:
-        name = getattr(i, 'name', '').lower()
+        name = getattr(i, 'name', '')
+        # BeautifulSoup4 sometimes results in 'tag' having attributes that have
+        # content 'None'
+        name = name.lower() if name is not None else ''
         if not name and not str(i).strip():
             pass
         elif name != 'li':
@@ -194,7 +203,11 @@ def do_list(tag):
 
 
 def do(tag):
-    name = getattr(tag, 'name', '').lower()
+    name = getattr(tag, 'name', None)
+    # BeautifulSoup4 sometimes results in 'tag' having attributes that have
+    # content 'None'
+    name = name.lower() if name is not None else ''
+
     if not name:
         text(tag)
     elif name == 'h1':
@@ -245,7 +258,7 @@ if len(sys.argv) != 3:
 
 infile = sys.argv[1]
 htmlfile = sys.argv[2]
-lines += open(infile).read().decode('utf8').split('\n')
+lines += open(infile, 'rb').read().decode('utf8').split('\n')
 
 # parse pandoc-style document headers (not part of markdown)
 g = re.match(r'^%\s+(.*?)\((.*?)\)\s+(.*)$', lines[0])
@@ -273,7 +286,12 @@ if AUTHOR:
 
 html = markdown.markdown(inp)
 open(htmlfile, 'w').write(html)
-soup = BeautifulSoup(html, convertEntities=BeautifulSoup.HTML_ENTITIES)
+
+if(bsver == 3):
+    soup = BeautifulSoup(html, convertEntities=BeautifulSoup.HTML_ENTITIES)
+elif(bsver == 4):
+    soup = BeautifulSoup(html, features = "html.parser")
+else: assert 0
 
 macro('.TH', PROD.upper(), SECTION, DATE, VENDOR, GROUPNAME)
 macro('.ad', 'l')  # left justified