from bs4 import BeautifulSoup

with open('a.html', 'r', encoding='utf-8') as a:
    soup = BeautifulSoup(a, 'html.parser')
    results = soup.findAll("td", { "class" : "td-text" })

    for result in results :
            if len(result['class']) == 1:
                print(result)

result = soup.findAll(
            lambda tag:tag.name == "td" and
            len(tag["class"]) == 1)
print(result)

html = '''
<tr class="tr-wrap">
  <td class="td-text td-text_bold">RU</td>
  <td class="td-text">800.00 руб.</td>
  <td class="td-text">990.00  руб.</td>
</tr>

<tr class="tr-wrap">
  <td class="td-text td-text_bold">BY</td>
  <td class="td-text">600.00 руб.</td>
  <td class="td-text">49.00 бел.руб.</td>
</tr>
'''

from bs4 import BeautifulSoup
root = BeautifulSoup(html, 'html.parser')
td_list = root.select('td[class=td-text]')
print([td.text for td in td_list])

['800.00 руб.', '990.00  руб.', '600.00 руб.', '49.00 бел.руб.']

html = '''
<tr class="tr-wrap">
  <td class="td-text td-text_bold">RU</td>
  <td class="td-text">800.00 руб.</td>
  <td class="td-text">990.00  руб.</td>
</tr>

<tr class="tr-wrap">
  <td class="td-text td-text_bold">BY</td>
  <td class="td-text">600.00 руб.</td>
  <td class="td-text">49.00 бел.руб.</td>
</tr>
'''

from bs4 import BeautifulSoup
soup = BeautifulSoup(html)
for td in soup.find_all('td', {'class':'td-text td-text_bold'}):
    td.attrs['class'] = ['td-text_bold']