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ARROW-9272: [C++][Python] Reduce complexity in python to arrow conversion

The original motivation for this patch was to reuse the same conversions path for both the scalars and arrays.

In my recent patch the scalars are converted from a single element list to a single element array then copied out from it.

On the long term we should convert them directly, perhaps with a more generic converter API, until that this patch aims to reduce code complexity without introducing any regressions.

I checked the produced binary size:

```console
# BEFORE
-rwxr-xr-x   1 kszucs  staff   2926832 Jun 29 23:07 libarrow_python.100.0.0.dylib

# AFTER
-rwxr-xr-x   1 kszucs  staff   2869136 Jun 29 23:06 libarrow_python.100.0.0.dylib
```

```
convert_builtins.ConvertPyListToArray.time_convert
==================== ========= ========
        type          before     after
-------------------- --------- --------
       int32          129±0ms   135±0m
       uint32         139±0ms   134±0m
       int64          132±0ms   139±0m
       uint64         135±0ms   132±0m
      float32         149±0ms   131±0m
      float64         143±0ms   141±0m
        bool          126±0ms   130±0m
      decimal         139±0ms   135±0m
       binary         145±0ms   130±0m
      binary10        136±0ms   130±0m
       ascii          145±0ms   136±0m
      unicode         148±0ms   149±0m
     int64 list       174±0ms   176±0m
       struct         164±0ms   177±0m
 struct from tuples   165±0ms   169±0m
==================== ========= ========

convert_builtins.InferPyListToArray.time_infer
============ ========= ========
    type      before    after
------------ --------- --------
   int64      157±0ms   158±0m
  float64     155±0ms   153±0m
    bool      155±0ms   154±0m
  decimal     249±0ms   233±0m
   binary     167±0ms   162±0m
   ascii      181±0ms   154±0m
  unicode     179±0ms   171±0m
 int64 list   188±0ms   192±0m
   struct     177±0ms   181±0m
============ ========= ========
```

I'm still running the benchmarks to prevent any performance regressions.

Closes #7584 from kszucs/py2arrow

Lead-authored-by: Krisztián Szűcs <szucs.krisztian@gmail.com>
Co-authored-by: Wes McKinney <wesm@apache.org>
Signed-off-by: Wes McKinney <wesm@apache.org>