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ARROW-9096: [Python] Pandas roundtrip with dtype="object" underlying numeric column index

- [x] closes ARROW-9096
- [x] tests added & passed

This PR fixes the roundtrip conversion for Pandas DataFrames whose column index is numeric but has `dtype=object`, such as
```
df = pd.DataFrame([1], columns=pd.Index([1], dtype=object))  # underlying int
df = pd.DataFrame([1], columns=pd.Index([1.1], dtype=object)) # underlying float
df = pd.DataFrame([1], columns=pd.Index([datetime(2018, 1, 1)], dtype='object')) # underlying datetime
```
https://issues.apache.org/jira/browse/ARROW-3651 largely solved the datetime variant of this problem (such that the conversion ran correctly excepting that the dtype after roundtrip did not match). With the current fix a roundtrip of the problematic DataFrames from ARROW-3651 returns the exact original frame.

Closes #7822 from arw2019/ARROW-9096

Authored-by: arw2019 <andrew.r.wieteska@gmail.com>
Signed-off-by: Wes McKinney <wesm@apache.org>