Removed subtransactions documentation

doc/index.rst

@@ -37,7 +37,6 @@ Learning more
    event
    historical_connections
    persistentclass
-   subtransactions
    utils
    developers
    changelog

doc/requirements.txt

 Sphinx
+# pygments 2.6 stops the support for python2
 pygments<2.6
 docutils
 ZODB

doc/subtransactions.rst

-=========================
-Subtransactions in ZODB 3
-=========================
-
-.. caution::
-    This document hasn't been reviewed since 2005
-    and is likely out of date.
-
-ZODB 3 provides limited support for subtransactions. Subtransactions
-are nested to *one* level. There are top-level transactions and
-subtransactions.  When a transaction is committed, a flag is passed
-indicating whether it is a subtransaction or a top-level transaction.
-Consider the following exampler commit calls:
-
-- ``commit()``
-
-  A regular top-level transaction is committed.
-
-- ``commit(1)``
-
-  A subtransaction is committed. There is now one subtransaction of
-  the current top-level transaction.
-
-- ``commit(1)``
-
-  A subtransaction is committed. There are now two subtransactions of
-  the current top-level transaction.
-
-- ``abort(1)``
-
-  A subtransaction is aborted. There are still two subtransactions of
-  the current top-level transaction; work done since the last
-  ``commit(1)`` call is discarded.
-
-- ``commit()``
-
-  We now commit a top-level transaction. The work done in the previous
-  two subtransactions *plus* work done since the last ``abort(1)`` call
-  is saved.
-
-- ``commit(1)``
-
-  A subtransaction is committed. There is now one subtransaction of
-  the current top-level transaction.
-
-- ``commit(1)``
-
-  A subtransaction is committed. There are now two subtransactions of
-  the current top-level transaction.
-
-- ``abort()``
-
-  We now abort a top-level transaction. We discard the work done in
-  the previous two subtransactions *plus* work done since the last
-  ``commit(1)`` call.