astropy.table writes int8 column in boolean in FITS output

### Description

When writing an astropy Table containing a np.int8 column into a FITS file, that column becomes boolean in the output
```
import numpy as np
from astropy.table import Table

t = Table()
t['a'] = np.array([-1, 0, 1, 2], dtype=np.int8)
print(t['a'].dtype) # int8
t.write('test.fits')

tt = Table.read('test.fits')
print(tt['a'].dtype) # bool
```

### System Details
Python 3.8.5 (default, Sep 4 2020, 02:22:02)
[Clang 10.0.0 ]
Numpy 1.20.3
astropy 4.2.1

Start with the Table.write and Table.read entry points using the reproducer in the issue, then trace the FITS output and input handling for np.int8 columns. Run the example with the listed Python, NumPy, and Astropy versions; done means the column remains int8 after writing to and reading from a FITS file.

Written by the indexing model from the issue text.