bpo-32494: Use gdbm_count for dbm_length if possible

[corona10]

By this PR, we can use gdbm_count without exporting a new public API.

I ran the benchmark and it shows noticeable performance enhancement.
This can be measured by invalidating cached value.

Benchmark 1

Run len(kv) after putting new value to invalidate the cache.

| Benchmark | bpo-32494-master | bpo-32494-proposed |
+===========+==================+==============================+
| bpo-32494 | 262 us | 42.2 us: 6.20x faster (-84%) |
+-----------+------------------+------------------------------+

import pyperf

runner = pyperf.Runner()
runner.timeit(name="[bpo-32494](https://bugs.python.org/issue32494)",
              stmt="""
ret = len(kv)
kv[f'key-{ret}'] = f'value-{ret}'
"""
              ,
              setup = """
import dbm.gnu as gdbm
from test.support import TESTFN
kv = gdbm.open(TESTFN, 'c')
for i in range(1000):
    kv[f'key-{i}'] = f'value-{i}'
"""
              )

Benchmark2

Remove caching code path to measure without putting new key/value.

-    if (dp->di_size < 0) {
+    if (1) {

+-----------+--------------------+-------------------------------+
| Benchmark | bpo-32494-master-1 | bpo-32494-proposed-1 |
+===========+====================+===============================+
| bpo-32494 | 109 us | 590 ns: 185.32x faster (-99%) |
+-----------+--------------------+-------------------------------+

import pyperf

runner = pyperf.Runner()
runner.timeit(name="[bpo-32494](https://bugs.python.org/issue32494)",
              stmt="""
ret = len(kv)
"""
              ,
              setup = """
import dbm.gnu as gdbm
from test.support import TESTFN
kv = gdbm.open(TESTFN, 'c')
for i in range(1000):
    kv[f'key-{i}'] = f'value-{i}'
"""
              )

https://bugs.python.org/issue32494

[corona10]

@pitrou Can you please take a look?

[pitrou]

Thank you for doing this. This looks mostly good, see below.

[bedevere-bot]

When you're done making the requested changes, leave the comment: I have made the requested changes; please review again.

[corona10]

Thank you for the review :)

I have made the requested changes; please review again

[bedevere-bot]

Thanks for making the requested changes!

@pitrou: please review the changes made to this pull request.

[pitrou]

+1. Thanks for the Py_ssize_t fix!