Adding read/readline() after write appends to EOF, not to the position

# Bug report

### Bug description:

```python
with open("testfile", "r+") as my_file:

    text = my_file.write("========")
    print(my_file.readline())

```
Instead of writing to the beginning of the file, the program writes to the EOF even if readline() is placed after write. 

### CPython versions tested on:

3.8, 3.9

### Operating systems tested on:

Windows