BUG: read_excel() using openpyxl engine header argument not working as expected
- [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.

---

#### Code Sample, a copy-pastable example
I am happy to provide the file I am using privately, offline if necessary. This would be necessary to directly reproduce this error. 
```
import pandas as pd
df = pd.read_excel(myfile.xlsx, engine='openpyxl', header=2)
```
Throws error:
```...ValueError: Passed header=2 but only 2 lines in file```

#### Problem description
I changed my `read_excel()` code to be now using `engine='openpyxl` with the 1.2 `read_excel()` changes, but the `header` argument is no longer working as it was before. I am trying to read in a .xlsx file that has 2 initial rows that should be skipped, and the 3rd row contains the headers. Thus, I am passing `header=2`, which should automatically skip the first two rows and use the 3rd (0-indexed) row as the headers.  However, I am getting the following error: `ValueError: Passed header=2 but only 2 lines in file`. I have confirmed multiple times that this file contains more than 2 lines - it contains thousands (happy to provide this file offline if necessary). I was able to previously run this code with the same file (before the `xlrd` deprecation, using the same header specification. With the openpyxl changes in 1.2, should we be using the `header` argument any differently? Thank you.

This is related to changes in https://github.com/pandas-dev/pandas/pull/35029.

#### Expected Output
Intended behavior would be that the .xlsx file is successfully read in as a dataframe using headers in row 3 and does not throw the above error.

#### Output of ``pd.show_versions()``
<details>

```
INSTALLED VERSIONS
------------------
commit           : None
python           : 3.8.6.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 19.6.0
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.0.3
numpy            : 1.19.4
pytz             : 2020.5
dateutil         : 2.8.1
pip              : 20.3.3
setuptools       : 50.3.2
Cython           : None
pytest           : 4.3.1
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : 7.19.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : 3.0.5
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : 4.3.1
pyxlsb           : None
s3fs             : 0.4.2
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : 2.0.1
xlwt             : None
xlsxwriter       : None
numba            : None
```
</details>