* bpo-29514: Check magic number for micro release

Add a dict importlib.util.EXPECTED_MAGIC_NUMBERS which
details the initial and expected pyc magic number for
each minor release. This gives a mechanism for users to
check if the magic number has changed within a release and
for a test to ensure procedure is followed if a change is
necessary.

Add a test to check the current MAGIC_NUMBER against the
expected number for the release if the current release is
at candidate or final level. On test failure, describe to
the developer the procedure for changing the magic number.

* Simplify magic number release test

Simplify the magic number release test by removing
EXPECTED_MAGIC_NUMBERS table and making the expected
magic number self-contained within the test.

BPO: 29514

* Improve magic number test execution and message

Improve the execution of the magic number test by
using skipUnless for alpha and beta releases, and
directly inheriting from unittest.TestCase rather than
using the machinery for the other tests. Also improve
the error message to explain the reason for caution in
changing the magic number.

BPO: 29514

Negative result should be returned only when an error is set.

* Remove conditional on free of `dps`, since `dps` is now allocated for
all versions of OpenSSL
* Remove call to `x509_check_ca` since it was only used to cache
the `crldp` field of the certificate
CRL_DIST_POINTS_free is available in all supported versions of OpenSSL
(recent 0.9.8+) and LibreSSL.

Replace the deprecated AC_TRY_RUN with AC_RUN_IFELSE. Also, standardize the
variable name and allow for caching of the result.

* Allow underscores in numeric literals in lib2to3.
* Stricter literal parsing for Python 3.6 in lib2to3.pgen2.tokenize.
* Add test case for underscores in literals in Python 3.

Replace `For object's ... ` with `For objects ...`

And fix the parameter name.

Reported by Lucio Ricardo Montero Valenzuela.

multiple times.  Writing to closed zip entry writer object now always produce
a ValueError.

The data contained in them is nonsensical

contextlib._GeneratorContextManager.__exit__ includes a special case to deal with
PEP 479 RuntimeErrors created when `StopIteration` is thrown into the context
manager body.

Previously this check was too permissive, and undid one level of chaining on *all*
RuntimeError instances, not just those that wrapped a StopIteration instance.

The reference to administrative data was confusing to readers,
so this simplifies the note to explain that deep copying may copy
more then you intended, such as data that you expected to be
shared between copies.

conn.set_trace_callback() shouldn't be called multiple times when the
schema is changing.

This has indirectly been fixed by using sqlite3_prepare_v2() in bpo-9303.

The original attempted fix missed an `isdir()` call in
`get_base_branch()`.