changing map annotation font size

**Description of the problem**

Setting
```
pygmt.config(FONT_ANNOT="6p")
```
appears to have no effect and setting it directly like this
```
fig.basemap(
region=region,
projection=projection,
frame=True,
font_annot="6p")
```
results in a parsing error.
```
GMTCLibError: Module 'basemap' failed with status code 71:
basemap [ERROR]: Malformed -f argument [ont_annot6p]
basemap [ERROR]: Option -f parsing failure. Correct syntax:
basemap [ERROR]: Offending option -font_annot6p
```
I may not see the forest for trees here... so any pointers are much appreciated

```
PyGMT information:
version: v0.2.1
System information:
python: 3.8.6 | packaged by conda-forge | (default, Oct 7 2020, 19:08:05) [GCC 7.5.0]
executable: /home/uliw/anaconda3/bin/python
machine: Linux-5.9.1-2-default-x86_64-with-glibc2.10
Dependency information:
numpy: 1.18.5
pandas: 1.1.5
xarray: 0.16.2
netCDF4: 1.5.5
packaging: 20.8
ghostscript: 9.53.3
gmt: 6.1.1
GMT library information:
binary dir: /home/uliw/anaconda3/bin
cores: 8
grid layout: rows
library path: /home/uliw/anaconda3/lib/libgmt.so
padding: 2
plugin dir: /home/uliw/anaconda3/lib/gmt/plugins
share dir: /home/uliw/anaconda3/share/gmt
version: 6.1.1
```
This is

**Full code that generated the error**

```python
PASTE CODE HERE
```

**Full error message**

```
PASTE ERROR MESSAGE HERE
```

**System information**

Please paste the output of `python -c "import pygmt; pygmt.show_versions()"`:

```
PASTE THE OUTPUT HERE
```