Fix a misleading assertion in test_pickling_with_module_import(). Only globals in the module are pickled

Support pickling modules with top-level import statements

See merge request inducer/pytools!12

module imports.

For portability to old Python versions, add a backported version
of importlib.

Add tools to find git revisions

See merge request inducer/pytools!11

Fix generate_numbered_unique_names() to respect the counter argument.

Closes #1

See merge request !10

Closes #1

Improve persistent dict documentation.

See merge request inducer/pytools!9

Nicer warning behavior for PersistentDict

See merge request !7

* Add a CacheCollision warning subclass.
* Track stack level so that warnings show user code line numbers.

Example:

Modifying line 287 of test_persistent_dict.py so that the warning
isn't captured by the test, we get this:

```
test_persistent_dict.py:287: CollisionWarning: pytools-test: key collision in cache at '/tmp/tmpq9p3i0b9' -- these are sufficiently unlikely that they're often indicative of a broken implementation of equality comparison
  pdict[key2]  # user code
```

The old behavior was:

```
/home/matt/src/pytools/pytools/persistent_dict.py:466: UserWarning: pytools-test: key collision in cache at '/tmp/tmpuk8js9jw' -- these are sufficiently unlikely that they're often indicative of a broken implementation of equality comparison
  % (self.identifier, self.container_dir))
```

Fix value used for testing store_if_not_present()

See merge request !6

Store if not present in persistent dicts

See merge request !5

PersistentDict cleanup; create WriteOncePersistentDict

See merge request inducer/pytools!4