py3: update mercurial changes for 6.3 release

requirements.txt

@@ -12,6 +12,7 b' hg-evolve==11.0.0'
 
 mercurial==6.3.2
 msgpack-python==0.5.6
+more-itertools==9.1.0
 
 pastedeploy==2.1.0
 pyramid==2.0.1

vcsserver/http_main.py

@@ -58,6 +58,7 b' from vcsserver.exceptions import HTTPRep'
 from vcsserver.lib.exc_tracking import store_exception
 from vcsserver.server import VcsServer
 
+strict_vcs = True
 
 git_import_err = None
 try:
@@ -66,6 +67,9 b' except ImportError as e:'
     GitFactory = None
     GitRemote = None
     git_import_err = e
+    if strict_vcs:
+        raise
+
 
 hg_import_err = None
 try:
@@ -74,6 +78,9 b' except ImportError as e:'
     MercurialFactory = None
     HgRemote = None
     hg_import_err = e
+    if strict_vcs:
+        raise
+
 
 svn_import_err = None
 try:
@@ -82,6 +89,8 b' except ImportError as e:'
     SubversionFactory = None
     SvnRemote = None
     svn_import_err = e
+    if strict_vcs:
+        raise
 
 
 def _is_request_chunked(environ):