Fix pandas FutureWarning: "Passing literal html to 'read_html' is deprecated"

This addresses ranaroussi#1685 (`institutional_holders`) and also `get_earnings_dates()`.

Pandas issue is found here:
pandas-dev/pandas#53767
and the change in code here:
https://github.com/pandas-dev/pandas/blob/5cedf87cccd77c7b4b6aaa64bfec98b32b512f68/pandas/io/html.py#L1238

As for legacy Python 2.7 support: `io.StringIO` seems to be supported in
the versions I tested. See https://docs.python.org/2/library/io.html

Author: mikez

yfinance/base.py

@@ -22,6 +22,7 @@
 from __future__ import print_function
 
 import datetime as _datetime
+from io import StringIO
 import json as _json
 import logging
 import time as _time
@@ -2090,7 +2091,7 @@ def get_earnings_dates(self, limit=12, proxy=None) -> Optional[pd.DataFrame]:
                                    "the issue. Thank you for your patience.")
 
             try:
-                data = pd.read_html(data)[0]
+                data = pd.read_html(StringIO(data))[0]
             except ValueError:
                 if page_offset == 0:
                     # Should not fail on first page

yfinance/scrapers/holders.py

@@ -1,3 +1,5 @@
+from io import StringIO
+
 import pandas as pd
 
 from yfinance.data import TickerData
@@ -36,7 +38,7 @@ def _scrape(self, proxy):
         ticker_url = f"{self._SCRAPE_URL_}/{self._data.ticker}"
         try:
             resp = self._data.cache_get(ticker_url + '/holders', proxy=proxy)
-            holders = pd.read_html(resp.text)
+            holders = pd.read_html(StringIO(resp.text))
         except Exception:
             holders = []