BUG: pandas merge suffixes accepting `set` can interchange right and left suffix order
- [x] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the latest version of pandas.

- [x] (optional) I have confirmed this bug exists on the master branch of pandas.

---

```python
df_b = pd.DataFrame(dict(p=[1,2,3], q=[2,3,4]))
df_a = pd.DataFrame(dict(p=[2,3,4], q=[1,2,3]))
pd.merge(left=df_b, right=df_a, on=['p'], how='outer', suffixes={'_b', '_a'})
```

#### Current output:
  | p | q_a | q_b
-- | -- | -- | --
0 | 1 | 2.0 | NaN
1 | 2 | 3.0 | 1.0
2 | 3 | 4.0 | 2.0
3 | 4 | NaN | 3.0

#### Expected Output:

  | p | q_b | q_a
-- | -- | -- | --
0 |1 | 2.0 | NaN
1 | 2 | 3.0 | 1.0
2 | 3 | 4.0 | 2.0
3 | 4 | NaN | 3.0


#### Problem description
This [line](https://github.com/pandas-dev/pandas/blob/837daf18d480cce18c25844c591c39da19437252/pandas/core/reshape/merge.py#L670
) is causing the issue:
```
lsuf, rsuf = self.suffixes
```
When you unpack set it returns elements in sorted order:
```
l, r = (2, 1); print(l, r) # l=2, r=1
l, r = [2, 1]; print(l, r) # l=2, r=1
l, r = {2, 1}; print(l, r) # l=1, r=2
```

#### Output of ``pd.show_versions()``

<details>
[INSTALLED VERSIONS
------------------
commit           : None
python           : 3.8.2.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.14.165-133.209.amzn2.x86_64
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.0.3
numpy            : 1.18.3
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 46.1.3.post20200330
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : None
IPython          : 7.13.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : 3.2.1
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : None
pyxlsb           : None
s3fs             : 0.4.2
scipy            : 1.4.1
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
numba            : None]
</details>
